At my school the savvy students wouldn't buy their books until after the first session of a course anyway, since the professors would often, in that first meeting, explain that some books listed in the syllabus as required were actually optional, or that they'd support some set of older editions of a book than the syllabus listed ("it lists the 5th edition, but it's OK if you get the 4th, and if all you can get ahold of is the 3rd, see me after class and we'll get something sorted out—but nothing older than that").