2. You will need the cash. Academic textbooks are not cheap and some of them can be re-sold for some much needed mulah.
3. Someone else might need them desperately. Think of those new students starting university. If you sell them your old textbooks, it could save them money they need for other important things, like beer or more highlighter pens.
4. It will help to save the planet. Facilitating the purchase of used textbooks mean that less need to be produced in the first place.
5. It will mean you have less to carry back home. You will have enough stuff to get back home after your course is finished. Do you really need to take all those textbooks with you too?