It's a very interesting model for me. Since I spent sometime many moons ago at one of the worlds largest online market places...one where you actually pay for what you get.
I've always found these "reverse auctions" a scam. Because its like a lottery as to whether you get anything, but its dressed up as a game of skill. I've never entered one and encourage no-one to try. The ratio of income to costs are sky high and there is some sharp practice around. It works because the punter believes headline figures of "£8.89 for a Mini Cooper". Sadly I suspect this affects the poorest and most vulnerable in society.
Incidentally there is one company best of the best, who do tickets for cars. They gross considerably more than the cost of the car...although that is a draw and doesn't rely on "skill".