I always bought used when possible. (Found some really good deals on half.com) I wouldn’t recommend renting unless you are sure that book will only be used one semester. Another thing to consider with new vs used is that many books come now have access codes for online content. You would want to make sure your teacher doesn’t plan to use this. You can always purchase an access code from the publisher but it won’t be cheap so by the time you pay for a used book and access code you may as well had bought a new book.