Exactly, the Amazon store that sells Gamestop cards would have to be a scammer. At which point, they get dropped as an Amazon store and you can have the person that purchased it, reverse his payment.
Shell would have to scam all Gamestop stores if they were to be reversing cards sold at Gamestop stores.
The only way it could reach the initial person getting the bitcoins is if you get the code e-mailed to you by the purchaser. Then they can just go to a Gamestop with that code before you and use it. Most people on Purse have things shipped so that you have a paper trail of a tracking code. And people also have reputation on Purse.
The only thing that would stop this from working is if Gamestop puts out a warning to all employees to not sell gift cards for Gamestop cards. Which is likely if it becomes widespread and popular.
But, Amazon also sells Southwest Airlines cards. So you can get 25% off on Southwest Airlines flights using Bitcoin as well.