If you do not pay, the seller can open an Unpaid Item Dispute. If you still do not pay, you get a Strike against your account, the seller gets his selling fees back (did you know sellers pay to sell here?) and you are blocked from leaving feedback.
Many sellers have automatic Blocks against bidders who have Strikes, so it becomes more difficult to bid and win.
If you have not yet won, you can politely ask the seller to Cancel your bid. Or you can Retract it yourself, although the reasons for this are limited.
http://offer.ebay.ca/ws/eBayISAPI.dll?RetractBidShow