What would happen is Bill (who works at the shop) gives you the game for £50 and then buys it himself on the day of release with his staff discount to balance the stocks. Simple. Nobody can prove Bill sold the game because there is no reciept or official record of transaction, Bill could just say you snuck into the stock room and stole it. Or that as you say it came directly or indirectly from an online retailer or mail order company.
In any case Bill is happy that the extra cash is worth the risk. And it's much easier for Crytek to prove that mail order companies and on-line retailers are dispatching the game before release.