Where can I find cheap used college books?

Does anyone know where I can go and find cheap used books for college around the Dallas or Richardson [Texas] area? if you do would you please be kind enough to let me know I need my books and they are really expensive at the bookstore of my school and they are used books and still expensive.Where can I find cheap used college books?
ebay or craigslist.com

duplicolor

mortal kombat dlc