Or maybe it's just outrageous to ask for $40 when it's clearly possible to sell it for $20. So you kind of punish the shop that asks for $40 because you see them as dishonest and morally repulsive. Sometimes you also have to pay attention to what behavior you encourage with your actions. Not only the immediate dollar value.

Why don't Christmas tree sellers sell the last, leftover Christmas trees for much cheaper, right before Christmas? Because then lots of people would just wait until that time and then buy it cheap. If buyers know that the seller will rather throw out the goods to the thrash rather than sell them cheaper then they will just casually buy the tree knowing that the price is stable and it's all fair. Short-sighted optimization would tell the seller to just sell the leftovers cheaper rather than throw them away.

Similarly, you may want to "send a message" to the $40 shop that you will rather drive a lot than participate in such an outrageous deal.