You can not cancel your bid after a sale has occurred because the integrity of the marketplace depends on it! We try to prevent market manipulation by ensuring bids are real, active and dependable. If you bid on something (even if it's not the highest bid), it can be accepted at any point if it becomes the highest bid.
If you were selling and thought you sold an item, you wouldn't want the buyer to be able to cancel, would you?
The good news is that StockX is a live marketplace. Once you receive the item, you can always resell on StockX.