I buy used--online, that is--sometimes I also buy the previous editions, as they're virtually the same. Obviously, you can't sell a previous edition back to the bookstore later, but really, what does it matter as they're just going to give you peanuts anyway--if they're even buying back your particular book(s) (I was always plagued with a prof who never turned in his book lists or something so they were never buying back my books anyway. We won't go into my opinion that if the bookstore sells a book, they should be required to buy it back).
I haven't tried re-selling my books on Amazon or anything, mostly because I've been too lazy to set up an account and try it, but I'm sure I could.