I wanted to edit my response since I was not incharge of the dept at the time the process was established. We now do receipt auction winners - we print the 'value' of the donated items in a booklet that is distributed as well as on bid sheets. We then subtract this value from the winning bid to determine the donation amount the winning bidder has made and this goes in the receipt field.
Though my current job does not include auctions (thank you, thank you, thank you) my previous employer held a large one very year. We would thank every winner with a letter that listed the item(s), the FMV and the amount paid. It was up to the winner and or a tax advisor to decied if there was any tax deductibility. The FMV was determined by our volunteer committee by adding up the actual value of the items in each "basket". As most of the items were new it was relatively easy to put a value on the baskets. However, many were listed a priceless (autographed baseballs, footballs, jerseys, a ride in the Goodyear Blimp, and stuff like that). I beleive the winner could not get a tax deduction for these.