Flim-flam men, the Duke and the King prey upon the gullible and the naive in The Adventures of Huckleberry Finn. They are "small-time" crooks who move from location to location before they are discovered, much like the traveling Bible salesman who follows the local papers obituaries.
While they are probably not as keen as Bernie Madoff or Charles Ponzi, for whom the term ponzi scheme is named, an plan which involved thousands of investors who paid previous investors' payments until there are not enough new investors to provide incoming funds; the King and the Duke, if living in modern times, may conduct what is known as "confidence games." In such situations, the victims believe in the good faith of the con artist because they are greedy, uneducated, or elderly and of diminshed mental capacity. The other accomplice acts as the "shill," who pretends to be a satisfied customer who has profited from his investments.
Most likely, nowadays they would be involved in some type of Internet fraud in which they get people to purchase some product or donate to some bogus cause.