Check studentlistings.com and bigwords.com

• Freeetextbooks.com/

Buy/ Rent:

• Chegg.com (for rentals + plant a tree!)
• Bookrenter.com/
• Half.com
• Amazon.com

Resell [http://collegecandy.com/2010/04/24/collegecandys-guide-to-selling-those-books/]