uh...the same way as buying textbooks from anywhere. you buy it, then they give it to you.
you can sell it back when you are done, assuming they are still accepting that textbook (new versions, etc can cause them to not accept it anymore) but you aren't getting the same amount of money.
it is the same as buying them from your school's store, except probably cheaper.
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I'm "kind of a big deal".
http://img.imgcake.com/cyclo/Cyclopngegpngre.png