According to ebay--

http://pages.ebay.com/help/buy/questions/retract-bid.html

If the actual account holder does not have the item for sale, I guess he won't be too angry if you don't actually buy the tractor he doesn't own.....if retracting turns out to be a hassle, I'd just let it go after informing the actual account holder.

Edit-Apparently the other guy can cancel your bid, too. Might look better on your bid history if you retract than if he cancels.