One of the most expensive parts of going to college can be purchasing the required textbooks needed for classes. Often one class will require multiple reading materials, and that adds up quickly when you are taking four or five classes.
Sometimes you can get away with borrowing the books from the library, but if your class is big then the one or two copies of the book that the library does have are going to be in high demand. Why not make it easier for a friend or classmate and offer them your old textbook for free? Or if you don't know anybody in need of it, donate it to the library.
You are helping a friend or classmate to both save money and be better prepared for class.
If the person you gave the textbook to is having difficulty in the class, offer to tutor them to help them better understand the material. Or you could start a program among your friends, working together to find free textbooks for those in need. Maybe even start looking outside your group of friends or campus; there are a number of international college students who need textbooks, too.
If you think you will need the textbook back in the future, just lend it to a friend for a semester. Or you could split the cost of a textbook with a classmate and share it for the duration of the class.
If you don't know of someone personally who is in need of a textbook but you still want to donate them, check out this site, it lists multiple organizations that accept used textbooks.