If there are more than 12 hours to go, the bidder can retract a bid by selecting the number of bids next to the price, and scroll down there is a link to do so. A valid reason needs to be selected and it is marked as a retraction on the bidders account.
They can also retract within 12 hours, IF it's within 1 hour of placing the bid.
If less than 12 hours (and +1 hour from bid placement), the seller needs to retract the bid.
Too many bid retractions on an account can see it sanctioned.
