When I was in college, my parents always advised me not to sell my old textbooks. They said that by keeping the textbooks I will always have that knowledge and information to go back and look on.
Being a full time student with limited funds, I ignored this advice and always sold my old textbooks back to the bookstore. Textbooks are expensive; it isn't easy to live with scarce funds. When college bookstores offered to buy back textbooks after the term ended, so naturally it made sense to sell back the textbook and have extra spending money.
8 years later, I regret doing that.
Without textbooks, it's become very difficult to remember all the lessons, insight, mathematical formulas, key words, and all the important information that I learned with the classes that required those books. If I still had my old textbooks, I could always go back to them and conveniently refresh my memory with the knowledge I attained from that class.
After college, your life will take lots of twists and turns. You may find yourself jumping from one job to the next. Having your old textbooks to always remember what you learned and studied in college will be a very convenient tool to make you more competitive in the job market when you're looking to improve your career.