Basically - you bid credits on items you want and if you're the winning bidder then yes (some people charge for shipping, a lot do not) - it goes to the highest bidder. And if you are not the highest bidder, you'll get your credits back.
You do have to earn credits though - you can complete offers on the site, buy credits, or put up your own auctions (after which you will receive the credits). It's a lot of fun once you get the hang of it - and you can get a lot of neat stuff! *I know I have :) * Good luck to you!