So what happens when you make a bad trade and you're left with less than what you owe the exchange?
Why would you owe the exchange anything?
How would the exchange cover the trades if not enough people were depositing?
Huh? In what sense does an exchange have to "cover" trades?
It would work like any other exchange except the necessary fee would only come after you withdraw your money from the exchange, say a flat 5%.
Try it and see how it does in the market. I think non-speculators wouldn't like to trade on such an exchange nearly as much, and speculators don't want to do business with just each other. But maybe I'm wrong.