Sayulita, consider very carefully how you structure the confirmations / time allowances.
Time delays should be minimized because any delay is market risk for one party.
It is true though that since once the deposit is made, the transaction is so to speak "binding", it's at least not a free option to any party (being short these options is my constant worry with anything like this).
Nice thought actually.
Any buyer can abuse the system doing the following:
1) Order bitcoin.
2) Wait for confirmation on price.
3) Wait an hour or two. If price goes up more than fee, honor the deal. If not, discard the deal without consequence.
There are no time delays or anything of the such.
This is how it works:
Buyer is given deposit information.
Buyer goes to bank and makes a deposit.
Timestamp on the deposit slip is matched against the price in the market at the minute the deposit was made to establish the price per BTC. There is no way to abuse the system as you have said.