Students in an undergraduate course often have no choice of a textbook for the subject and are expected to buy the book recommended by the instructor. The expectations can be advisory or mandatory. Given the degree of control that an instructor has over their undergraduate students, especially freshmen and sophomores, is it ethical to use a textbook authored by the instructor in an undergraduate course? Does such requirement not constitute a conflict of interest?
P.S. I understand that practically this is done times and again, but I would like to sense the general feelings.