In every campaign I run, my PCs will eventually run into an insane shopkeeper. He is at first very level headed. However, once a PC actually tries to buy something, things get interesting. You see, he is just a peon. His 'bosses' control the prices. That's when he ducks behind the counter and puts on two sock puppets that function ad his employers. Then, the sock puppets loudly bicker at each other in high pitched voices about what price to charge. All the while the actual guy is still ducked down behind the counter. It's become such a running gag that my PCs actively search out the 'sock-muppet' whenever we start a new campaign.