It means, at the time you bought, you thought the painting was worth $50 million. Later, when you discover it's not, you'll be less likely to buy anything else from that art dealer. Same as anything else in life. People keep buying ketchup because ketchup remains valuable to them, even at $2 a bottle.

Of course not. Had you known it was a forgery, you wouldn't have spent $50 million. That would apply just as well to buying a bottle of Heinz ketchup tainted with rat poison.

But people know what's in Heinz. They know what it tastes like. They keep buying it at the price Heinz and their grocer charges.