abebooks.com

If you didn't know about this great site already, go there. Many used books with very nominal wear and tear. I just ordered Introduction to Operations Research for 4.75 USD, including shipping. The last book I got from one of their hosted dealers had a few red pen marks that didn't obscure the text at all ... it was a steal. On average, a book there costs a little more than a combo meal at Wendy's and is definitely a better investment in the long run. I recommend it to everyone.