I agree with Reez on some points.
Yes, all profs have different preferences, so you can't assume that because the leading casebook in one area is the one your prof will use.
Some schools actually do release book lists early. Others don't. With some you have to wait till you get your schedule and section assignments which is usually during orientation, so you don't have time to order them online.
I think you can get some of your casebooks online cheaper if you have the time to order them and the time to shop around. Hit Amazon, ebay, and if you have the ISBN, use search by it instead of the title. I have gotten some real steals on casebooks and study aids online. There are some students that plan on reselling them and don't mark them up at all. Others are not that heavily marked up. The key is to order from someplace that is very specific about the condition of the book, not someone who just says "like new" or "good" because their idea of like new and good may not be the same as yours or the standards set forth on, for example, Amazon.
For my first semester, I couldn't order my books online because I didn't get my class assignments until orientation. From then forward, though, I ordered online and have cut my expenses in half, easily.
Also, I recommend buying new or at least unmarked books your first semester. Once you get a semester under your belt, you may find used with some highlighting is not that bad.
I also always buy whatever the edition the prof recommends, usually the most current. Some of my friends buy older versions, but a lot of my profs give assignments by page numbers, so they don't always correspond. And don't read in advance. Not all profs start at page 1. Most jump around and some even skip sections