The Collectable Cat

A famous art collector was walking through a city he was visiting when he noticed a mangy cat lapping milk from a saucer in the doorway of a store. He did a double take when he saw what the cat was drinking from.
He knew that the saucer was extremely old and very valuable, so he walked casually into the store and offered to buy the cat for two dollars.
The store owner replied, "I'm sorry, but the cat isn't for sale."
The collector said, "Please, I need a hungry cat around the house to catch mice. I'll pay you twenty dollars for that cat." And the owner said "Sold," and handed over the cat.
The collector, carefully nonchalant, continued, "Hey, for the twenty bucks I wonder if you could throw in that old saucer. The cat's used to it and it'll save me from having to get a dish."
To which the owner replied, "Sorry buddy, but that's my lucky saucer. So far this week I've sold twenty-three cats."