Generally, no. Once you have made your bid you are agreeing to purchase the computer up to that amount. There is a confirmations screen that asks you to confirm your bid, so mistakes should occur rarely. Because of Proxy Bidding you may not have to pay your maximum bid amount.
Bid retractions are allowed if you make a mistake when bidding. For example, adding an extra zero on your bid. The mechanism for retracting is to contact Anna Vargas at Liquis.