I'm sure they've looked into it. The problem (as I see it, and I'm not a lawyer) is that 1.) there is no law against used game sales, so Gamestop and others aren't criminally guilty and 2.) Gamestop apparently isn't in breach of any contract with publishers by doing this (it's the gamers that are in breach, but suing your customers is almost never a good idea, and in this case it's an incredibly stupid one).