Are they in good condition? That measn, no ripped pages, no hgihlighter marks, no sticky pages, and that the book won't fall apart the moment you begin to read it. However, normal wear and tear like those little white spots that appear because of openning and closing the book so many times and some folded pages are totally OK.
If they're in good condition, then do some research on Amazon and see how much they're selling the book for. Then, because it's used, deduct, maybe, a dollar or less, and you should be good.
