Placing a bid on an eBay auction at the last possible moment, or in some cases even the last second, is a process commonly known as sniping. Although some eBay members question the practice of sniping, this process is not against any of the rules established by eBay. While some members may argue that sniping is unfair to bidders and actually keeps the auction prices down, others find this practice a fun and exhilarating way to get the final bid in for an item just before an auction closes.
Instructions:
1) Log in to your eBay account. It is best to log on to the Internet and sign in to your eBay account at least ten minutes before the auction is set to end. This will allow you some extra time should you have a problem with your Internet connection or accessing your eBay account.
2) Open the eBay Web page to the item you plan to bid on. You will use this page to keep up with the bidding and how much time is left on the auction.
3) Open a second Web page with the same auction on it. You will use this page to place your bid.
4) Type in the maximum bid amount that you wish to place on the second web page. Click on "Place Bid," but do not click on "Confirm" until there are only ten seconds left before the auction ends.
5) Refresh the first Web page if it does not seem to be updating the bids and time left. Adjust your bid on the second open page if you discover you have been outbid and continue refreshing the page to keep bids and time current.
6) Watch the countdown time and place your bid. Ebay will count down the final minutes and seconds of the auction. Confirm your final bid at the ten-second mark to allow time for the bid to process.
Source - eHow