Your site & TV advert are a disgrace - you are essentially conning not very savvy people out of their hard-earned cash.
Anyone with 1/2 a brain cell can see its a con - essentially you are charging people for the privilege of bidding in auctions they are almost certainly never going to win & then when they have spent £100, £200 or whatever you give them the opportunity to buy overpriced goods with that credit.
Why can't you just be honest that that's how it works - because you're making a fortune obviously!
Won't be long though before you go the same way as Wonga & either people wake up to the con or Consumer organisations step in to change your business model & require you to be honest.