Textbooks for Rent

In my inbox, I just got a promotion for TextBookFlix, which allows you to rent textbooks for a semester, and, according to the website, save 55% to 65% on your textbook bills. As a grad student, this is quite tempting; however, is this a sustainable business model? In my experience, at least, the turnover for textbooks (and indeed, any books used for a class) tends to be rather high -- new professors come in, some classes cycle through different professors each semester, some classes are only offered occasionally, if at all, etc. Add to this the fact that professors are usually idiosyncratic about their textbook choices, and it's hard to see how this could work, at least from the point of view of the bookseller. (For the book buyer, the ultimate fate of books rented is arguably not important.) I guess that for introductory courses that everyone has to take, there is plenty of money to be made in renting textbooks; but for advanced courses, its profitability has to taper off.