If you really want to you can back out of it. eBay auctions for real estate are nonbinding.
http://pages.ebay.com/help/policies/non-binding-bid.html
So you can contact the seller and explain that you didn't really want to pay that much and give him another number. Be willing to walk away from the deal if you go that route but you might also get to buy at your lower offer. It's worth a try, IMO. Just keep in mind that if you bargain-hunt you might make enemies. People don't like selling things for less than they think it's worth.
In the future, remember you can negotiate before the auction too. I bought my Carlsbad Inn week by simply offering the ebay seller what I wanted to pay. He accepted and we had a deal - no auction. No need to snipe or worry about other bidders, just swooped in there before it got hot.