Con got conned

The seller will say that the painting is by someone notable such as Leonardo or whatever. In reality, someone else just painted a very close replica of the actual painting.

There's a great book by Edward Dolnick, called "The Forger's Spell". It's all about how Han Van Meegeren sold fake paintings to the Nazis for ridiculous sums. He netted somewhere around $30 million of today's dollars.