Same scheme as guns4pennies.
For our previous discussion of that particular site, give this thread a read:
guns4pennies - The Firing Line
As an aside...
When I was just looking up some information on the GunBiddler site, I noticed in the "Latest Winner" section, that some one had just won a Ruger SR40 for a displayed price of $13.46. That means at least 1,345 bids had been placed. Since the cheapest price to purchase a bid is $0.60, each... that means the site took in a minimum of $807 on that auction. But, since most people buy smaller bid packages (up to $0.94 / bid), they may have taken in as much as $1,264.30 on the auction.
If you only place one bid and win... it's a good deal. But, that's usually not the case.
And, once you buy your "bid package", your money is gone. You won't be getting it back when you're outbid.