honestly the best we've done is checking amazon to see if they are buying back-they send the shipping stuff by email and give us the payment in the form of a gift card. since the best prices on buying for dd are also on amazon we just use the cards for the next quarter's purchases.
p.s. don't toss a textbook of none of the sites are buying it, hold on to it for a few quarters-we've found that one of dd's older books comes up every once in awhile (esp. if it's the kind of class colleges only offer once a year).