If you're a tax-exempt organization, you really need to have actual values for everything. To the extent that anyone pays greater than the FMV for an item, they could deduct the excess of what they paid over the FMV on their tax return. And, you have to give them a receipt if that excess $$ amount is greater than a certain number (can't remember if it's $75 or $250, and I don't have time to dig for it at the moment, but you should be able to find it on the IRS website). While it may seem unlikely that anyone would pay that much over what it's worth (most are in it for the bargains), it depends what the item is--it's not unheard of.
The auctions I have seen make it VERY clear what the value is if they have one. Many people are looking for a deal, so they figure out what they are willing to bid based on what the stated value is.
I personally will bid up to 40% of the item's value. When looking at a donation from a photographer, knowing what he charges normally would make me willing to bid more than if it where from Sears Photography. (No offense to Sears intended.)
I'm going to push my group to publicize it everywhere. Also you may draw more people to your auction by advertising "$40,000+ in items."
Many of our auction items are theme baskets which I don't have exact values for, or art objects made by the children (of course, priceless), but I was wondering how best to publicize and then post at the auction the value of items with clear value. For example, a parent who is an artist is donating a personal portrait. He typically charges $1200 for this; should this be in the booklet? A dentist contributed professional teeth bleaching, value $550. I guess the question is, should I give values for things that I know the actual value for? In the auction booklet?