
I love GameStop. I have a few friends that work there, so I get to learn a few things about GS' rules & policies.
I like to have my games complete too, so I've run into your problem before. What I do is when I already own the game incomplete and I see a case & manual later in the store, I ask the store manager if I can have the case & manual. GS doesn't make money off the case & manual, they make money off selling the game. That's the reason why the case and manuals are out on the shelves and "complete" games aren't more expensive than "incomplete" games. You save yourself the trouble of having to rebuy and return/resale a game you already own. If the employees in at the store know you as a loyal customer, then you shouldn't have any problems. They're not breaking any laws or anything letting you have the case & manual. They do have to make a new fake case for the game they hadn't sold yet though and that takes all of 3min.