Sale or Not for Sale

An art collector is walking through a big pet store where he notices a
dirty bad looking cat licking greedily the milk from a dish in the doorway of
a store. He takes a very close look at the scene and he notices that the
dish is extremely old and very valuable. He walks casually into the store and
offers to buy the cat for two dollars.

The store owner replies, "I'm sorry, but the cat isn't for sale."

The collector says, "Please, I need a hungry cat around the house to catch
mice. I'll pay you twenty dollars for that cat.

And the owner says "Sold," and passes over the cat to the collector.

"Hey, for the twenty bucks I wonder if you could throw in that old dish.
This cat is used to it and it will save me having to get another dish."

And the owner says, "Sorry my friend, but that's my lucky dish. So far
this week I've sold seventy cheap cats for very high price."