I usually use Amazon and Chegg. I've had really good luck selling items back on Amazon at the end of the semester and Chegg has been really cheap for the books I need(sciences) and it takes away the depression of trying to sell books back and getting $5 for something I spend $150 on
Wherever possible, do what I did in undergrad. Go to class the first day and get the syllabus. If it has exactly the pages/chapters you need, you're golden. Go buy the book, photocopy what you need, then return immediately for a full refund. Tell them you made a mistake and bought the wrong edition, whatever. Most of my books cost $ 100 but photocopies of what I actually needed might have cost me $20