The rules are stated here, and they are fairly simple: (there was a url link here which no longer exists)
APUG does not require that a price be stated, though it would be a good idea to do so, since that's what people are accustomed to here. There is no prohibition against auctions or any other kind of arrangement that people want to try within the classifieds, as long as everyone is honest and up front about it.
We do ask that people not use the classifieds to promote eBay or other off-site auctions, but we're even fairly lenient about that, so long as APUG members have had a reasonable opportunity to purchase the item before it goes to auction, and if it does, the seller should not create a new thread with the auction link, but just add it as a comment to the existing thread.