Are you bidding on an auction? You are allowed to Retract your bid if you immediately enter the correct bid.
http://offer.ebay.ca/ws/eBayISAPI.dll?RetractBidShow
If you have bought the item, whether in auction or as a Buy It Now, contact the seller and explain your dilemma. Most will (unhappily) allow a Mutual Cancellation. But they don't have to fix your mistakes, so leave glowing feedback.
