Most universities have a policy that if the professor authored a textbook he himself assigns, then he is required to return a comparable proportion of the royal

Most universities have a policy that if the professor authored a textbook he himself assigns, then he is required to return a comparable proportion of the royalties. Perhaps he was forcing you to buy brand new so that he didn't overpay the royalties back (i.e. they may have had a simple formula based on the number of students, regardless of whether you bought or not).