When I was in university, I bought my books. Almost all of them hardcover with a few paperback. I noticed that one of the books I had to buy, was written by the professor teaching that same class.
I felt that since he had essentially a captive audience, we get a dollar or two off the price. It didn't happen.
I did ask the bookstore, not owned by the university, and they said that publisher contracts wouldn't allow it.
I asked him, and he said it couldn't be done. And smirked.
I'm sure there are legal reasons why it couldn't be done. But instead of explaning them, he just smirked. Like I was an idiot. I'm not, I'm willing to listen.