I don`t think PC World have done this deliberately and perhaps if the assistant had come back and said "Sorry we don`t have any but we have one that hasn`t been repackaged..." you might have a different slant on the story. On two occasions I`ve accepted a pre-owned product, one a cable router, without any problems with the end product. In both cases they had been re-shrink wrapped - I tend to throw the packaging away myself. ;o)
From their terms and conditions:
We try to make sure that [email protected] goods always remain available for purchase until you arrive at the store, however we cannot guarantee this. For this reason we do not take any payment from you until you come to our store.
Which I think is fair enough. Stock control systems aren`t infallible. You saved over £18 after all.