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.