I don't do a lot of Ebay buying but I use Auctionsniper when I do. I tried another one before that; can't remember the name. IME they all seem to work. Frankly I don't understand why everyone doesn't snipe.

Another benefit of sniping is you can change your mind and cancel a snipe eg. if you discover you do in fact have a seatpost in the stash and don't need that one you were about to bid on. Bidgroups are great too - set multiple snipes for like items, and when you win one (or the number you specify) the others are automatically cancelled.