Sellers/Listing Services Addendum EFFECTIVE DATE: July 14, 2009

Exhibit 10.6

Sellers/Listing Services Addendum

EFFECTIVE DATE: July 14, 2009

EquipNet, Inc. (“EquipNet”) provides a service (the “Services”) that matches buyers with sellers of used equipment (the “Equipment”). As a result of its limited role, EquipNet has no control over the quality, compatibility, safety or legality of the Equipment offered for sale.

This Sellers/Listing Services Addendum (this “Addendum”), when executed by both parties, will become a part of and incorporated into the EquipNet, Inc. General Terms of Service, the current version of which can be found at www.equipnet.com (the “General Terms”) and this Addendum shall supersede the EquipNet’s standard Seller’s Terms of Service, the current version of which can be found at www.equipnet.com. Accordingly, the term “Agreement” as used in the General Terms shall mean the General Terms plus this Addendum (plus, if Company is also a Buyer, EquipNet’s standard Buyer’s Terms of Service, the current version of which can be found at www.equipnet.com). All capitalized terms used and not defined in this Addendum will have the meanings given to them in the General Terms.

Upon execution of this Addendum by the parties, Company shall be considered a “Seller Member” and all transactions will be deemed specifically and directly between the seller and the buyer of the Equipment. In addition to EquipNet’s Services, EquipNet may also provide other services, such as, on-site, project management, consignment, software or valuation services (the “Management Services”).

1.0 Posting Equipment.

1.1 Company is providing EquipNet the exclusive right to market and sell the listed Equipment (a “Listing”), described on the Equipment attached to this Addendum as Schedules A and B.

1.2 EquipNet may under this Addendum use a variety of sales channels including MarketPlace^tm, Live Auctions, On-line Auctions, or other third party listing services.

1.3 The sales period (“Sales Period”) will be for a period from the Effective Date through October 31 ^,,2009. The Sales Period may be extended beyond October 31, 2009 only by written consent of Company, which consent shall not be unreasonably withheld; provided, however, that any such extension shall be subject to termination pursuant to Section 5.0. The parties agree that the auction of certain of the Equipment shall be held during the month of September 2009.

1.4 Company is responsible for the accuracy of all information provided by Company with regard to the Equipment.

1.5 Company will disclose to EquipNet whether any Equipment has been used for processing or manufacturing any product containing materials that would restrict or otherwise affect subsequent use of the Equipment for any purpose.

1.6 EquipNet reserves the right to reject any Listing or to request modifications to the Listing to conform to the requirements of the Services.

1.7 Company hereby grants to EquipNet a non-exclusive, royalty-free license during the term of this Agreement to use Company’s Trademarks and logo worldwide; provided that they are used solely in connection with the promotion, marketing and sale of the Equipment or the Services generally and in accordance with any specification that Company provides to EquipNet concerning style, color and typeface. EquipNet will have the right, at its own expense, to: (i) place advertisements in trade and other publications and issue press release(s) describing its services to Company; and (ii) refer to Member and the Services in its promotional materials. Company will have the right to review and approve advertisements and promotional materials referencing Company (or any of its Trademarks); provided that such approval will not be unreasonably withheld, delayed or conditioned.

2.0 Company’s Representations/Warranties.

Company represents and warrants that: (a) all Listing information is complete and accurate; (b) Company has all right, title and interest in, or the exclusive right to sell, the Equipment; (c) the Equipment is free from any lien or encumbrance; (d) the Equipment will not contain any hazardous or bio-hazardous materials at the time of sale and (e) the Equipment and Listing information does not infringe any third party intellectual property right or violate any law or regulation.

3.0 Payment.

Company authorizes EquipNet to collect payment on its behalf. EquipNet will collect in its own bank account the proceeds paid by the buyers. Company should not release the Equipment to any buyer until payment confirmation has been received from EquipNet. All non-refundable deposits that are collected will be applied to the purchase of Equipment. EquipNet will retain 15% of all deposits if buyer forfeits such deposits. In addition to the Seller’s Fee listed in Schedule A, EquipNet may receive additional fees from the Buyer, such as service fees for rigging, removal, shipping and/or buyer’s premiums related to the sale of Company’s Equipment. These charges will be the sole responsibility of the buyer and will be invoiced directly to the buyer by EquipNet.

3.1 Expenses for Management Services.

Subject to Schedule A, all expenses of EquipNet such as inventorying, project management, and cataloging will be the sole responsibility of EquipNet so long as Company facility where the existing Equipment is currently located is at least available for

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storage of the Equipment through October 31, 2009. Expenses such as removal work, shipping preparation and/or freight will be the responsibility of the buyers of the Equipment. If the Equipment must be moved from its current location on or before October 31, 2009, other than in connection with a sale of the Equipment, then the Company will be responsible for expenses associated with such move, such as removal work, shipping preparation and/or freight, (to the local EquipNet warehouse in San Diego, CA) and associated storage costs through October 31, 2009; provided, however, if the Equipment must be moved after October 31, 2009, then EquipNet will be responsible for all associated expenses, such as removal work, shipping preparation and/or freight and storage costs, associated with such move after October 31, 2009 unless the Company agrees to an alternative arrangement with EquipNet.

4.0 Terms of Sale.

4.1 Terms of Sale. COMPANY WILL BE SOLELY RESPONSIBLE FOR ALL TERMS AND CONDITIONS DIFFERENT FROM THOSE BUYER TERMS ALREADY ACCEPTED AS PART OF MEMBERSHIP REGISTRATION BY BUYER SUCH AS, BUT NOT LIMITED TO, CHANGES IN REGARD TO WARRANTY TERMS (IF ANY), AVAILABILITY AND FIRM RELEASE DATES, LOCATION CHANGES, AND OTHER DISCLAIMERS. COMPANY AGREES THAT ALL TERMS AND CONDITIONS GOVERNING ANY TRANSACTION ARE EXCLUSIVELY BETWEEN MEMBERS ACTING AS BUYERS AND SELLERS. COMPANY HEREBY ACKNOWLEDGES THAT EQUIPNET AND ITS OFFICERS, DIRECTORS, STOCKHOLDERS, AGENTS, SUBSIDIARIES AND EMPLOYEES ASSUME NO LIABILITY WITH RESPECT TO THE QUALITY OR QUANTITY OF EQUIPMENT SOLD OR OFFERED FOR SALE OR FOR ANY LOSS OR DAMAGE TO ANY EQUIPMENT. EQUIPNET WILL NOT BE RESPONSIBLE FOR ANY BREACH, BY EITHER THE BUYER OR SELLER, OF THE TERMS OF ANY TRANSACTION. TO THE MAXIMUM EXTENT PERMITTED BY APPLICABLE LAW, COMPANY HEREBY WAIVES, RELEASES AND DISCHARGES EQUIPNET (AND ITS OFFICERS, DIRECTORS, STOCKHOLDERS, AGENTS, SUBSIDIARIES AND EMPLOYEES) FROM ANY CLAIMS, LOSSES, DAMAGES AND EXPENSES, WHETHER DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL OR PUNITIVE OF EVERY KIND AND NATURE, KNOWN AND UNKNOWN, DISCLOSED AND UNDISCLOSED, ARISING OUT OF OR IN ANY WAY CONNECTED TO ANY TRANSACTION, INCLUDING DISPUTES WITH OTHER MEMBERS. In the event Company is a resident of California, Company waives California Civil Code §1542 which states, “A general release does not extend to claims which the creditor does not know or suspect to exist in his favor at the time of executing the release, which if known by him must have materially affected his settlement with the debtor.”

4.2 Company Indemnity. Given that the transaction is directly between the Company and Buyer Member, Company agrees to defend, indemnify, and hold EquipNet harmless from and against any damages, liabilities, costs and expenses (including attorneys’ fees) arising out of or resulting from use of the Services by Company, its employees, agents, customers or independent contractors, or in any way connected with Company’s possession, sale, use or transfer of Equipment (including, without limitation, personal injury or property damage related thereto), the failure of Equipment while in a Buyer’s possession, failure to consummate a sale, the breach of any of Company’s warranties hereunder, any product or item processed or manufactured with or by Equipment, or any violation of any terms or conditions of this Addendum.

5.0 Term and Termination.

5.1 Term. This Addendum will commence on the Effective Date and will remain in effect through the termination of the Sales Period, as such may be extended pursuant to Section 1.3 (the “Term”), unless terminated earlier as per Section 5.2.

5.2 Termination. This Addendum may be terminated prior to the expiration of the Term by (a) EquipNet, immediately in the event that it ceases to operate the Services; (b) by either party, in the event the other party materially breaches a provision of this Addendum and the breaching party fails to cure such breach within thirty (30) calendar days of the receipt of written notice of such breach from the non-breaching party.

5.3 Effects of Termination. Upon termination of this Addendum for any reason, all rights and obligations of the parties hereunder which by their express terms or nature are intended to survive any termination or expiration of this Addendum will survive. In addition, upon any such termination, as applicable, (i) EquipNet shall pay Company any excess of Sales Proceeds (as defined in Schedule A), received by EquipNet prior to termination, above Payments (as defined in Schedule A) made to Company or (ii) Company shall pay EquipNet any excess of Payments (as defined in Schedule A) made to Company, prior to termination, above Sales Proceeds (as defined in Schedule A), received by EquipNet prior to termination. All monies must be returned to the appropriate party within five (5) business days of termination.

IN WITNESS WHEREOF, the parties hereto have executed this Addendum as an instrument under seal as of the date first set forth above.

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SCHEDULE A

EQUIPMENT SCHEDULE

to that certain Sellers/Listing Services Addendum, dated as of July 14, 2009, by and between EquipNet, Inc., (“EquipNet”) and Metabasis Therapeutics, Inc. (“Company”).

Equipment Description:

The equipment listed on the attached in Schedule B.

Sales Proceeds, Expenses, Fees & Conditions:

“Sales Proceeds” for purposes of this Agreement are defined as the Gross Value of the Equipment sold, where “Gross Value” is the dollar amount collected from the Buyer for the sale of the Equipment, not including any direct Buyer expenses such as buyer premium, shipping, preparation, taxes or duties. Such amounts will be deemed “collected” for the purposes of this Agreement after the equipment is released and title is transferred to the Buyer.

During the Sales Period, provided the Addendum has not been terminated pursuant to Section 5.2 thereof, EquipNet shall pay to Company, by the dates set forth below, the following amounts (each, a “Payment” and collectively, the “Payments”):

EquipNet guarantees to pay Company at least $1,500,000 in the aggregate in accordance with the schedule set forth below (the “Minimum Guarantee”), provided the addendum has not been terminated pursuant to section 5.2 therof

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The Seller Fee shall be the sum of the following:

The above percentages are cumulative and apply to all Sales Proceeds during the Sales Period in excess of $1,550,000. For example, Sales Proceeds of $2,250,000 will provide EquipNet with 28% of $700,000 ($2,250,000 - $1,550,000), or $196,000. See table attached as Schedule C for a flow of funds to EquipNet and the Company.

The Company and EquipNet agree to the following, in addition to any of their obligations under the Addendum and the General Terms, during the entire Sales Period:

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Pricing Approval:

The following describes the process for EquipNet accepting offers on the Equipment listed in this Addendum. This procedure will be used to establish acceptable resale values.

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The exact time frames will be established by EquipNet based on the continuing availability of the space and/or other factors that impact what will be the most appropriate timeframe.

Sales Tax Processing

EquipNet will assess for all MarketPlace sales where the buyer is located in California at the current sales tax rate for the buyer’s “ship to” destination. If a Buyer can provide a valid tax exemption certificate or tangible proof that the equipment was shipped outside of California then we will credit the appropriate tax off of their invoice. All sales tax for MP sales will be remitted to Metabasis for reporting to the state of California. EquipNet will deliver a completed sales report each month that will detail the state each buyer is from, and whether tax was collected for that sale or if not then copies of exemption certificates and proof of export. For auction sales, EquipNet will use its subsidiary EquipNet Auctions, Inc. to perform the same process as above, except that EquipNet Auctions will report the sales and remit the sales tax to the state. As a licensed auctioneer, EquipNet Auctions are required to collect and remit the tax associated with sales from auctions and the Company would not have any liability associated with the Auctions. For sake of extra clarity and certainty, EquipNet Auctions will fully indemnify Company for any liability resulting from the collection and remittance of sales tax related to sales performed by EquipNet Auctions.

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SCHEDULE B

Equipment list

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