Silly man. Yiannis is a legend among ultra runners (Dean says as much in his book) and doesn't need to do this sort of stuff. Plus, the bet is one sided (Dean has nothing to gain and everything to lose). YK could beat Dean over almost any road or track distance (marathon and up. Trail is a different matter) and they both know it.
I'm no fan of Dean, but I don't believe that he has, personally, claimed to be the greatest. He has certainly let other people make that claim for him without correcting them, but accusing him of defrauding people and "selling lies" is a little over the top. It just makes YK look petty, IMHO.
According to Wikipedia, Karnazes is “the world’s most famous ultra marathon runner.”