yes you are right sasan
I agree with real world senarios
but real world senario of e-shop is more like order by phone shops.
in order by phone you may order some thing and when they bring your staff they may tell you
we are sorry we run out of X and they may also tell you that they have bringed you Y instead!
I mean they usually don't let customer go! :D
or better say they use optimistic locking and they use "excuse me" or "replacement strategy" to handle exceptions! ;-)