This article is from the eBay FAQ, by William B. Shaw email@example.com with numerous contributions by others.
Although no one can guarantee that someone else isn't willing to pay a
higher price for any given item, there are a few common sense strategies
that can help prevent losing auctions to a sniper.
a. Whenever you place your bid, bid your ABSOLUTE maximum. If you think you
have determined your max, ask yourself if you would have bid $1 more to win
in the face of a competing bid. If you are willing to spend another dollar
to win, you haven't found your max. Remember that the amount of your bid is
far more important than the timing of your bid. A sniper can only win if
they bid higher than your maximum bid.
b. Become a sniper yourself and use it to your advantage. Just remember that
you only have time for one bid and the current high bidder may have entered
a bid much higher than what is currently showing. Therefore, bid your max.
c. Recognize that very few items are truly unique. If an item is on eBay, it
will probably come up again. Move on and catch it next time around.