I realize that but they allow me to return them up to 2 weeks after instruction starts. So I buy them now (hopefully getting them cheap) and then return them once the class begins if I don't need it (they give a full refund).
IceMan is absolutely correct. You can always get the book later or borrow a friend's if you need it once or twice. I don't buy books unless I'm gonna do assignments out of it on a regular basis. I've saved hundreds, if not thousands so far.