Normally, the donor sets the value of donations in order to claim a charitable contribution deduction.  If the donor intends to claim a deduction of more than $500 for gifts of things other than cash and securities, the donor must include a Form 8283 with the tax return claiming the deduction. The form must describe the gift, but need not be signed by the receiving charity..  Generally, if the claim is for more than $5000, the form must include a summary of an appraisal by an independent appraiser.  (See Ready Reference Page: “IRS Requires Substantiation of Contributions”)

No contribution deduction is allowed for gifts of services or facilities.  If gifts are made by a corporation, special rules apply to gifts of inventory.  (See Ready Reference Page: “Corporations Have Special Rules on Contributions”)

The one exception to the general rule of having the donor estimate the value occurs with charity auctions.  In the case of the charity auction, the charity is obligated to tell the purchaser of the item the fair market value of the item so that the purchaser can claim a charitable contribution deduction if the price is more than the value.  In that case, the purchaser can claim a deduction of the difference.  If the price is less than the fair market value, of course, the purchaser cannot claim a deduction.  Since the donor of the item for auction may be claiming a deduction for the value of the item (although the donor cannot claim an appreciated value of personal property that will be auctioned off), it makes sense to talk with the donor and try to agree on a fair market value, but the charity has a legal obligation to make an estimate to tell the purchaser. 

Many popular auction items are service items, like a week at a vacation condo, for which the donor gets no deduction, and the purchaser is able to claim a contribution deduction only if, and to the extent that, the price is more than the estimated value of the service.