If I were a professor who wrote a textbook based on a class I teach, of course I'd make it the textbook I use and make everyone buy a copy. Chances are that the textbook's material closely matches what I actually want to teach in the class. Why wouldn't I want to use my own path through the material? No other textbook is going to match identically or synthesize the material in my way.
I agree. So you would sell, assuming it was an introductory-course textbook for a popular class (best-case scenario), what- a few hundred copies? And that's under the conditions above.
My mechanics teacher wrote our textbook. We had about 10 people in our class. How much money do you think he made? Even if a few other schools adopted it (a VERY difficult task unless you work on your network of friends- which doesn't always go over well in academia), we're talking small money.
He could consult a few hours a year for an aerospace company or something and make a lot more. That's my point.
