Isn't their websites where you can take other peoples bets? You know.....as opposed to making the bets yourself. Kinda like being the bookie. You make an agrreement to take someones wager or not.
I think you are thinking of a peer to peer site like Matchbook but that closed to US players in 2011.
Considering that model of sports betting cuts out the sportsbooks and the government pretty significantly, I wouldn't expect to see it be available in the US anytime soon.