a common mistake book sellers make is getting stuck with out-dated college/high school books. it's super easy to look at a chart for 2 seconds and assume that you'll be able to sell it at a higher price... when in reality there's a new edition eating up the market and amazon hasn't listed it yet. here's a good example.

while amazon didn't indicate a new edition, i was able to tell by looking at the drastic increase in used offers combined with the drastic drop in used price. this is 6th edition, turns out the 7th edition just came out.