As a general rule, you can’t retract or cancel a bid. When you place a bid, you agree to pay for the item if you’re  the winning bidder.

However, honest mistakes sometimes occur. You can retract a bid if you have a valid reason for doing so as defined by eBay policy and you retract the bid within the required time period.

Valid reasons for retracting a bid include:

1. You accidentally entered the wrong bid amount due to a typographical error.

2. The item’s description changed significantly after you entered your last bid.

3. You can’t reach the seller by telephone or email.

Changing your mind is not a valid reason for retracting a bid. Before bidding, be sure to carefully review the listing.

If you meet the conditions and understand the results of doing so, fill out a Bid Retraction form. Learn more:
https://signin.ebay.com/ws/eBayISAPI.dll?SignIn&UsingSSL=1&pUserId=&co_partnerId=2&siteid=0&ru=http%3A%2F%2Foffer.ebay.com%2Fws%2FeBayISAPI.dll%3FRetractBidShow%26guest%3D1&pageType=563