Stay away from those places. You have to buy the right to bid, and that doesn't mean your bid will win. You purchase a certain amount of "bids" so if you buy 10 and use them for that one auction, you have to buy more bids. So you end up paying for the bids and the product.
Also, the winning bid isn't the amount that you pay, it just means that you won and they will tell you a price thats acceptable for the product and you can choose to buy it or not. I don't know if thats the case for all those auctions, but there was one that worked that way.
ETA I just looked at their site and under the FAQ is said that the last bid is the price you pay, so thats not the one I was talking about.