Hello 'cardbaron',
Yes, you can, but it is considered pretty poor form. It's just not the done thing.
You see, there are only a couple acceptable reasons for retracting, say, bidding $999 when you
meant to bid $9.99
So have a look through the following link, and you will see a Bid Retraction Form you can use:
http://pages.ebay.ca/help/buy/bid-retract.html