No one is going to be making a fake Madden 99 cart for the gbc, it's always going to be the heavy hitters like pokemon or atlus games. Idk how they prevent fakes on both ends tbh, from taking in fakes to selling them as well. But if they're going to sell them they better be responsible about it.
In any case, isn't it illegal for them to not ensure they're legit? Selling counterfeits is definitely illegal
That's true, but won't stop you from buying fakes, just confirm that you did. The best thing you can do is to educate yourself on what a fake looks like, especially if you're going to be buying pokemon emerald for $200.