The catch is that you pay to bid regardless of if you win an item or not.
So... say each bid increases by 10p and costs £1 to make that bid (I've not checked the exact amounts)
An Item with a retail value of £1000 goes up starting at 0 and it is eventually sold to the successful bidder for £500 would result in the website getting £5000 in bidding fees for a £1000 item plus the £500 it was sold for. (so they've been paid £5500 for a £1000 item)
If you were lucky enough to put one bid in at £500 pounds and won the item you're onto a winner as everyone else who hasn't won it has paid for the item and more with their bid charges, if you bid £400 pounds you've got nothing and it cost you £1