Wanna really piss them off?
1> Hang around "pretend" to shop. Wait for someone to come in and try to sell their games. When they get the ol "best we can do is 10" say to the customer "oh! I was looking for that game! I will give you 30 bucks!"
2> Same scenario, except customer comes in to buy a used ps3. "Dude, I have two used ones I'm selling for 140 bucks, complete with all the cables and two controllers!"
They hate when people do these things, but gamers gotta have gamer's backs