For sufficiently fungible products, the "Market Maker as Exchange" model works really well. What you'd basically have is high spreads for retail investors, and

For sufficiently fungible products, the "Market Maker as Exchange" model works really well. What you'd basically have is high spreads for retail investors, and lower spreads for institutional investors. So your consumer-facing company would make a spread and then flatten its exposure by trading with other market-makers. This gives the "Exchange" more flexibility than Mt. Gox has. It can offer more competitive prices when it's worthwhile, and pull back if trading with retail customers gives it too much net exposure. That's better than having a fixed fee for matching people with market-makers. This is a rough approximation of how forex works.