That's called plagiarism. Fan fiction is something you do for fun. Professional fiction is marketing work you have a legal right or permission to market. Trying to profit off of something that belongs to somebody else, without their permission, is stealing. It's anything but professional, because it violates the standards of professional conduct. And it's anything but fannish, because it disrespects the property and its creators by trying to usurp it as one's own. It's just greedy.