Many textbooks come with license keys specifically so you can’t buy used books anymore.
Students who do buy a used book are typically coerced into buying the license key to do digital homework, or whatever other reason.
When I was teaching during a graduate fellowship I accepted paper assignments for students who bought used books, or who shared books with other students. The Pearson representative complained and I was given a talking to by the graduate student dean about consistency or some such nonsense. Eventually I would be reassigned to teach classes that didn’t involve digital components.
We called it the textbook industrial complex.