You may be able to cancel a normal withdrawal after 30 minutes, as long as the seller has not sent the item yet. Auction withdrawals cannot be cancelled.
To cancel, expand the section underneath the trade in the menu on the right:
If cancellation is available, you will see the Cancel Purchase button:
You must have Trade Tracking enabled for this option to work. Cancellation is not available for every withdrawal. If you do not see the button, you cannot cancel that trade.
If a buyer cancels their purchase, the item will be automatically relisted.