When you create an auction you can enter a reserve price. If,
at the end of the auction, no bidder has entered a bid higher than the
reserve price you do not have to sell the item.
The advantage
of a reserve price is that you might be able to see what people are
really willing to spend, and you don't have to sell if they don't meet
your price.
The disadvantage
is that some bidders are turned off by reserve prices; they feel if
they make a bid, they should be able to buy the item at that price.
Reserve auctions can be frustrating for the bidder.