For me it's a deal breaker, always. Partly because I like to have the actual box, and partly because if someone can't be bothered to keep the original case then how can I be expected to trust them with taking good care of the item (whether it's a game or something else) that they're trying to sell me? There's a reason that things are packaged, after all, not just for the sake of looking pretty. GBA games are a very rare exception but I almost never play GBA games anyway, I either emulate the ones I already own or used to own, or else buy the updated DS versions if applicable, or go without. I'll only buy a GBA game if it's something I can't legitimately get in another format and have never previously bought and owned (since I don't see an issue with emulating something I used to own as long as I bought the original legitimately, since that's not exactly pirating if the company already honestly had my money for it at some stage, right?).