It's not a scam... but it can feel like one if you don't know what you're doing. You can loose money if you're bidding on something that you plan to purchase anyway... you're just taking a shot at a better deal. If I want a laptop and know I'm going to buy it no matter what and I lose the betting... then I can buy it off the site anyway and get all the money I bet with credited towards my purchase.
BE SMART people. If you're not good at math and you think everything comes easy to you then don't go to this site. You'll just end up feeling and looking like an $#*! hole.