While there's no Gamestop in my country, there are dedicated gaming stores and all of them know a lot about gaming, so I rarely have this problem.
Although Wallmarts are a whole different story, I remember one time that I wanted to see how much the guy knew about video games, so I asked him that if I could see the case of Mario Galaxy for the PS3, at that particular Wallamart the games are in different sections (PS3, 360, Wii, 3DS, etc) so the guy went to the PS3 section and start looking for the game (he obviously wasn't going to find it), he even looked very deep just to try to find, after a while he just told me "Sorry, we don't have the game". While the guy was searching I was looking at Mario Galaxy in the Wii section. Haha I'm a horrible person :)
Nintendo and PC gamer
