I dont really understand your how that should work. Make a proof-of-concept site and we have something to talk about.

meanwhile, have a look at bitcoin.de , they dont ask for a money deposit, only a BTC deposit from the seller. So in case the buyer doesnt actually pay, nothing is lost and and the BTC can be returned to the seller; on the other side, the BTC from the seller is frozen when a transaction starts, so when a buyer did transfer the money but the seller doesnt confirm, then the exchange-owners can always check and release the frozen amount of BTC.
It is slow (because of banks), but it works.