Quoted from Rarehero:Perhaps the slight difference is this: Marvel owns all their characters that convention artists sell sketches and prints of. Perhaps they don’t go after them because they’re too big to care, perhaps cuz it would be seen as a backlash against the fans. In any case - it’s their IP, their call to make.
Comic artist can sell their original art they did for a book.
They can also create and sell original art with licensed characters if it is a one off.
They can NOT create mass produced prints of a licensed character(s) and sell them.