The owner looks at the TV set, then looks at the stoner, and says, "I don't sell anything to potheads."
So the stoner tells the owner that he'll quit smoking pot and will come back the next week to buy the TV.
A week or so later, the stoner comes back and says, "I quit smoking pot. Now, how much for that TV set in the window?"
And the owner replies, "I told you I don't sell to potheads!" So the stoner leaves again.
He comes back a month or so later and says, "How much for that TV?"
The owner says, "I'm not going to tell you again, I don't sell anything to potheads!"
The stoner looks back at the owner and asks, "How can you tell I'm a pothead?"
The owner looks back and says, "Because that's a microwave."
Source: Alcoholic Humor