If this http://offer.ebay.ca/ws/eBayISAPI.dll?RetractBidShow is not useful, then ask the seller to cancel your bid.
If he does not do so, when the auction is completed, ask him for a Mutual Cancellation. This is the fastest way to end a purchase.
The MC allows you to leave feedback and Detailed Seller Ratings. The most effective feedback is calm and factual.