As a general rule, you can't retract or cancel a bid. Once you place a bid, you agree to pay for the item if you're the winning bidder. However, honest mistakes sometimes occur.
If you want to retract a bid:
Review the conditions and time restrictions for canceling a bid in an auction-style listing.
If you feel you have a valid reason for canceling a bid, fill out the Bid Retractions form.
If you want to cancel a Best Offer, use the Best Offer Cancellation form.