You have a fantastic idea here, I just don't think you are executing the sports side of it the right way. Why do you have two contracts for each side of the same game? You are effectively cutting the liquidity pool in half for no good reason that I can think of.

Do you remember tradesports? They had the right idea. You need two contracts per game. One with a straight up winner, and one with a spread. For example:

GiantsVsBears.Bears 
GiantsVsBears.Bears -9.5

That's it. If people want the Bears, they go long... If they want the Giants, they go short. It's simple and it basically doubles the market depth per contract compared to the system you are using now.

I would love to see your site take off and become the next betfair, but tradesports style.