I’ve been in that particular situation several times. When I was an Econ major, not only were the school’s own professors high flyers, the visiting professors were literally some of the most famous academics in their fields- the guy I took 101 with was literally the man whose book was the #1 Econ textbook in the English speaking world. My Anti-trust prof gave us a week off when he testified at the Time-Warner merger hearings in DC.Yes, it happens -- maybe the professor wrote the best book available on a specific topic, after all -- but I certainly wouldn't feel comfortable peddling a book I'd written myself to students.
And our Philosophy department was pretty much the same.
(Yes, I was a dual major.)
IME, the only one who abused the power of his syllabus had been the #2 man under U Thant at the UN, and put 6 of his books on our Poli-Sci 101 reading list, of which we only really used 1 or 2. I mean, the info in the books was stunning, but he got an extra $400 or so out of every student he taught that class to.