I was told by a local college (the Math Department Chair) that professors are not permitted to tell students that they can get their books for less online. I find that disgusting.
Equally disgusting are professors whose required textbooks are the ones he/she wrote. I remember that from back in the '80s.
Most professors spill the beans.
As an engineering student I hated that the textbook companies put out a new edition of the core calc / diff EQ textbooks every two years (you could see it in the front page material) which were always identical to prior editions except for the problems, which they changed slightly. Because the profs assigned those problems for homework, you had to buy the full priced version, the used one was useless even though the content was identical because math is always the same. So shady.