If it's illegal, I'd report the seller to Amazon.

It is not illegal, the printed remark is not binding. The publishers send large numbers of unrequested books to instructors, trying to get them to adopt the book. there have been rulings that the instructor who receives unsolicited merchandise is under no obligation to adhere to these requests.

I would consider it an ethics issue if an instructor *requested* a book and then sold it - but the reality is that, if the instructor requests *one* specific textbook to see if he wants to adopt it, he will then also receive ten other books he never wanted to inspect. And those then end up at textbook sellers, in pristine condition.