The Pedigree Dog
A stranger walks into a bar with a dog trailing him on a leash. He asks if the owner can watch his dog for a few minutes while he places a bet or attends to a business deal. While the stranger is gone, a second con artist arrives and notices the dog. He claims to be an expert on dog breeding, and says that this dog is worth hundreds, if not thousands of dollars. He asks the owner if the dog is for sale because he'll pay top dollar. The entire scam hinges on the bar owner's greed. The assumption is he'll see the chance to buy the dog from the unsuspecting owner for a low price, then sell the valuable dog to this "expert breeder." He tells the dog expert to come back later, then offers to buy the dog when its owner returns. The dog's owner sells it, but the "expert" never comes back to buy it. The two con artists walk away with a few hundred dollars, and the bar owner gets stuck with a "mutt." In Neil Gaiman's novel "American Gods," two characters discuss this con using a violin instead of a dog.