I have the impression that discussion on this topic is discouraged, but I am curious about people's attitudes too. The short answer is, there is nothing you can do. No matter your intentions, you violated Nintendo's copyright (i.e., broke the law) by modifying the ROM in the first place. The person selling cartridges with your modified ROM is essentially on the same ground as you, legally. It comes down to honor among thieves, and there is none.
My point of view is that people selling your work (Nintendo's work, with your modifications) should give you a portion of the sales, because they profit from work you have done. But they don't have to, so they don't.