Prop bet over the internet -- third party to hold the money?

[AvivaSimplex]

I want to make a prop bet with another 2+2 poster for several thousand dollars. I've never met this person in real life. Given that it's just a stranger on the internet, I think if I win the chances he won't pay me are quite high. I'm sure he thinks the same of me. The solution is, we need an independent third party to hold the money, evaluate who won, and pay the winner. Is there some website somewhere that can do this?