This is one of the valid reasons for Bid Retractions.
Go Here:
http://offer.ebay.ca/ws/eBayISAPI.dll?RetractBidShow
If you have already won, use Contact Seller to explain your problem and ask very politely for a Mutual Cancellation. If this is granted, agree immediately and leave Positive Feedback like "Thank you for helping me with this transaction. Five Star Seller!"
If you have already won, the seller is also entitled to start an Unpaid Item Dispute. If you do not pay, you will get a Strike. Many, perhaps most, sellers have automatic Blocks against bidders with Strikes.