Salma, You are always able to return a purchased or rented textbook within the first week of your class start date as long as you have your receipt and the book is in it's original condition. After the first week you may return with proof that you dropped the course as long as you have the original receipt and it's within the add/drop period.
Once you buy, you're set in the deal. Every book for each class is different however. And if pages are blatantly torn out or such they may let you. I recommend giving them a call!
You have to call and check. I think it depends
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