Another affordable option is to swap/exchange the textbooks you already have for the ones you need.
Here are a few ways to do so:
Please Note: Many book exchange services require you to match with someone who has the book you need and is willing to take your book in return. Do some research on the service you plan to use so that you won't be waiting for a perfect match to get the books you need for class.
If you bought your textbook through the Gettysburg College Bookstore, you can sell it back for 50% of what you paid if the book meets certain conditions. If you bought your textbook(s) from another seller, there are also a growing number of online retailers which will buy them from you.
Here are a few:
Remember: regardless of where you're planning on selling your textbooks, make sure to take good care of them throughout the semester. A book's value is directly related to its quality and while normal wear-and-tear is expected, any highlighting, margin notes, coffee stains, or dog-eared pages can detract from its value.