Ha this is my guide to how much to sell your textbook!

Discontinued Editions
100% to 50% off depending on how different they are +10% increase (for bargaining purposes)

Good quality
almost perfect quality and no dirt (did u use it?)
30% off +10% increase (for bargaining purposes)

Moderate quality
a used look with some notes and scribblings
50% off +10% increase (for bargaining purposes)

Bad quality
very used possibly missing the front page! (what the hell were you doing with it!) many highlighting and scribblings
100% to 75% off depending on how cui it is +10% increase (for bargaining purposes)

*Include extra notes, exam papers and other stuff to jack up the price or to counter attack requests to lower the price
and remember the buyer is always as desperate or more desperate than you!