1. How do you know that the price increase shown on the site actually corresponds to the number of bids?
(Very few of these sites show 'current bidder' or similar information)
2. You PAY for each bid, not the winning price. As you have no way of knowing if it's your bid that 'wins' the auction or not, these 'auctions' are considered a lottery in many countries, and actually banned in a few.