Not as uncommon as you think. Writers will add book purchases into their contracts, so if you get stuck with a bunch of books you had to buy to get them there, it seems probable that they were pressing some students to buy some of the ones the school may have been forced to buy. It is the latest and greatest industry practice to inflate their sales numbers. A silly one IMHO as I would rather you just paid me the money and cut my publisher out of it, but I guess being able to say you sold 100K copies instead of 12,000 copies makes you more desirable to do speeches, sit on panels, etc etc etc.