Conflict of interest: Forced to buy a book from my professor who published it

I am student and the dean published a textbook (new edition every year, $250) which is published only in small amount and sold only in a store on campus (whose owner is his friend...). last time, he came to a class taught by another professor and told us: "You are required to buy the book" and "It is sold only in this store and we will figure out who didn't buy it". Maybe checking the names of people who bought. Which is illegal I guess. The book itself is not more useful than other books on the same topic. For me this is Conflict of interest and even threatening us. the program is very challenging (because they admitted much more student that they can handle, they are failing students quickly). What can I do? This seems like mafia activity to stole our money...