You can retract a bid (not buy out) if the auction has more than 12 hours left before the listing ends. When you do this, all your previous other bids for this auction will also be eliminated. So be sure to bid again if you are correcting a bidding error that you made.

To retract your bid, click the bidding tab in the members area. Find the auction you wish to remove the bid on and click the remove bid option.  

Note: There is no equivalent for bid retraction in Buy Out listings, or for listings where the bidding has ended. Once you've confirmed your purchase, you are obligated to pay the seller.