One lecturer at a Polytechnic I worked for made his students buy his book. Well, a photocopy actually, done without payment from him by the Poly's Copy Services.
Other lecturers got "gifts" from publishers for requiring or at least recommending the publisher's books.
The amount of corruption in higher education is quite astonishing - you only have to look at the prices of required/recommended books compared with actual good, classics to realise this.
Is it corruption, or just an established business model for poorly paid educators to increase their revenues?
They were not so poorly paid - I was a senior analyst/programmer (and did some teaching), quite reasonably compensated, and the lecturers would get quite a bit more than me.
But if you want to substitute "established business model" for "corruption", go ahead. I must say that not all of them were bad.
Its both
I started studying at UNISA in the mid-90s. It was a distance learning university, with fees literally 1/10th that of a in-person university. They had more current students than all the rest of the SA universities combined.
Roughly half the textbooks required were published by UNISA press, with authors being the lecturers themselves. With one exception (Delphi programming), all the books published by UNISA press were free with the course.
It's astounding that +3 decades later, it is still not profitable for any other university to do this!