How can anyone tell you whether some hack job will work? Maybe some of them work fine. Regardless, you'd be smart to avoid buying any, and you can do it if you try. The chance that you'll be supporting someone shady and that you'll get a horribly put together piece of shit are not worth it. Besides, even if you don't care about any of that, a bootleg MVS cart has literally no value to most people. Only a few games might possibly be valuable to some people, like maybe a game that never came out to begin with, but if you think like that, chances are you'll find your name in a "sellers to be wary of" thread.
