The problem starts with the sales strategy of textbook publishers. Due to the spread of second hand selling, large publishers have rationalized increasing the margins they make per sale. They are able to sell at high margins by making even slightly old versions of textbooks obsolete. This is achieved by creating one use access codes and attempting to reorganize and change the content of the book, so that only the new edition of the book can be used in a class.
Big book publishers also try to make the purchase of new textbooks necessary to enroll in a class. They do this by having colleges charge students a fee, and then provide them with the textbook. They claim that this will allow students to save money by having the school purchase the books wholesale. This tactic is actually done to limit consumer options, and forces students to buy new textbooks unnecessarily.
To summarize, big publishers use their market power to make used books obsolete or even get rid of consumer choice all together. This is possible through the creation of one time use access codes and getting universities to charge their students for pre purchased new books. It also hurts our business by limiting the demand for used books.