Definition of fair market value from the IRS:
IRS Publication 561 - Determining the Value of Donated Property: "Fair market value. Fair market value (FMV) is the price that property would sell for on the open market. It is the price that would be agreed on between a willing buyer and a willing seller, with neither being required to act, and both having reasonable knowledge of the relevant facts. If you put a restriction on the use of property you donate, the FMV must reflect that restriction."
Treas. Reg. 20.2031-1(b)- Valuation of property in general: "The fair market value is the price at which the property would change hands between a willing buyer and a willing seller, neither being under any compulsion to buy or to sell and both having reasonable knowledge of relevant facts." This is in the estate context, but is generally the definition used for tax purposes.
Info related to charitable donations in connection with receipt of goods / services:
Treas. Reg. 1.170A-1(h) Payment in exchange for consideration—
(1) Burden on taxpayer to show that all or part of payment is a charitable contribution or gift. No part of a payment that a taxpayer makes to or for the use of an organization described in section 170(c) that is in consideration for (as defined in § 1.170A-13(f)(6)) goods or services (as defined in § 1.170A-13(f)(5)) is a contribution or gift within the meaning of section 170(c) unless the taxpayer—
(i) Intends to make a payment in an amount that exceeds the fair market value of the goods or services; and (ii) Makes a payment in an amount that exceeds the fair market value of the goods or services.
IRS Publication 526 - Charitable Contributions: "If you pay more than fair market value to a qualified organization for goods or services, the excess may be a charitable contribution. For the excess amount to qualify, you must pay it with the intent to make a charitable contribution."
General info from IRS Website - Charity Auctions: "Donors who purchase items at a charity auction may claim a charitable contribution deduction for the excess of the purchase price paid for an item over its fair market value. The donor must be able to show, however, that he or she knew that the value of the item was less than the amount paid. For example, a charity may publish a catalog, given to each person who attends an auction, providing a good faith estimate of items that will be available for bidding. Assuming the donor has no reason to doubt the accuracy of the published estimate, if he or she pays more than the published value, the difference between the amount paid and the published value may constitute a charitable contribution deduction."http://www.irs.gov/Charities-&-Non-Profits/Charitable-Organizations/Charity-Auctions