In reply to nufkin:
> But almost invariably one would pay at the time of ordering; does that mean if a (probably small) retailer doesn't have software that accounts for levels of stock in real time they'd have to honour the contract regardless?
No, you enter card details, you have not paid. Read what I posted.
> (Say, using the OP example, there's one pair of boots left, but two people place an order close enough for a website not to be able to update and make them unavailable)
The retailer gets an alert an order has arrived, they then have the opportunity to go to shelf and make sure it's not been sold by a colleague in the shop, or if stock is low, double check it's there. Then you take the money, the contract is formed, or put another way, you have done a deal.
If after this you renege, you are breaking the deal and trust.
Should the OP have a heart attack over a few quid, no, but it is shitty behavior by the business, and I doubt the owner knows. A small business person values their reputation and good name above all else, or they should.