One of my guiding principles as a college teacher is to never make rules I can't enforce. This clearly falls under that category. Even if I was concerned about this, there's nothing I can do about it. If a student wants to sell her notes to one of these companies all she has to do is not write her name on them, and I'll never know who did it. Prohibiting it would just make me look like a moron who doesn't understand the limits of his own authority.