What can/should you do if you buy a fake copy of a game? [closed]

If the retailer isn't at fault (which it could be; for example an employee who switches the merchandise with a copy in the back room), it could be a dodgy distributor. Either way, the best party to chase this up would probably be the manager at the retailer. They can work "up the supply chain" from there. Anyone else is unlikely to want to or be able to help.