Many students do but if you do I'm astounded! Say you spent $60.- on your textbook. You used it for countless hours, perhaps you marked and customized it (a good thing to do), you know it intimately, you are able to work with it efficiently, and go back to it and save many more hours using it to deepen your understanding of this particular area of mathematics, for the fun of it or because you have to, and you sell your book for $20.- or so? How much would you sell your mother for?
Fine print, your comments, more links, Peter Alfeld, PA1UM