I think the idea is really great. The problems that I can see are a lot of bets like that are made relatively non-chalantly, and the process you are implementing is a good amount of work. I think most bets like that wouldnt even happen if you had to go through that amount of leg work. I've had that happen numerous times, and the repercussions of not being an "honorable" (can't think of another word to describe it) gambler is even worst than the $5-$50 the offender might lose.

That being said there is still a lot of upside if people buy into it. But , in my opinion, any friend that isn't going to uphold his side of a bet on a sports game is just an asshole