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.