My advice:
YOU DONT NEED TO BUY TEXTBOOKS BEFORE CLASS STARTS! This is a common mistake. Your first few lectures are usually an introduction to the course, what it's about, etc. and doesn't require textbook homework right away so you don't need the textbook yet until a few weeks into classes.
Keep in mind, especially for elective courses you might take, you may not like the class once you attend the first lecture and want to switch out of it. You can do this during the add/drop period at the start of school where you get the change to drop courses and add a different one. So if you end up not liking the course Day 1, but you already bought the textbook.. you're stuck with it! (if you bought it used), or you'll have to refund it.
Textbook publishers usually come out with new 'editions' every few years as you said... and sometimes are sneaky by switching around chapters and pages! But for the most part you'll be ok getting the used edition of the previous year's textbook. However, before doing so I would first ask the professor during your first lecture class.
We have a load of great tips and advice on this topic in our ADVICE section, check these out:
How To Find What Textbooks You Need
Textbook Buying Guide
How to find the textbook list for your classes
And to answer your question on where to buy used textbooks...
Where to buy/sell textbooks (Updated for 2012)