P.S. What's to stop me exploiting this with other sellers, submitting a 'buy' order, then waiting several days to complete it, and getting the coins at an extreme discount vs the current market spot price? My morals and scruples say 'no, don't do that', but it would be good (I think) if your service codified it.
A contact is not a contract to buy at a set price. The seller is free to cancel the deal and make you create a new one.
the whole idea of locking in a price doesn't make sense as long as the deal is not binding, which it isn't (anyone can run at any time).
I would love a way to advertise the fact that I make the price on the spot right before the cash is exchanged. The price of a "locked-in" deal could be constantly recalculated using the formula (buyer would have to be able to enter amount in EUR or BTC). I would even use the site (sms) to make the transactions and pay the fee if that was implemented since it would spare me from having to look up mtgox price, convert to EUR, add my margin all while explaining the process to the customer.
Wouldn't this be the solution? Kangasbros could have his fee and the traders would have a fair deal with no doubts.