check this link from ebay:
you get negative feedback when you place a bid and win, but you wouldn't buy it. but if the seller has provided his/her contact details, then maybe you can tell him/her that you're canceling your bid.
here's a little tip though. if you're sure that you really want to get that item, don't bid immediately. check how long the listing is, and maybe you can wait until it's just a day or even hours away from ending. the thing is, there might be bids already, so you'd have to bid higher, but there's a bigger chance of you getting the item. what i usually do is wait until the item ends in 2 hours or so, then place my bid. by then, i'm already sure that i do want to get that item.
hope this helps. happy buying!