In the interest of readability, the target in these cons is named Mark. The con artist is Carl, and his accomplice (if there is one) is Anna.
THE FIDDLE GAME
Dressed as a poor musician, Anna buys something cheap from Mark's restaurant. When the bill arrives, Anna tells Mark she left her wallet elsewhere. She offers to leave her old, beat-up fiddle as collateral, then leaves.
Later, Carl enters the restaurant and spies the fiddle. After asking where Mark got it, Carl says the fiddle is a classic and offers $50,000 for it. Mark can't sell it, of course (it's not his), so Carl leaves his business card and tells him to tell the owner of the fiddle of his offer. Carl leaves.
Anna finally returns with her wallet. If Mark dutifully passes on the message, the con fails (though with no repercussions for Carl or Anna). But Mark is greedy and desperate for $50,000. He offers to buy Anna's fiddle. Anna, of course, refuses, as the fiddle is her work, but she is finally convinced to sell it for a modest sum, say $500.
At that point, Anna and Carl disappear, with a profit of $500, less the cost of the piece-of-junk fiddle now in Mark's hands.
THE FALSE GOOD SAMARITAN
Anna mugs Mark, but Carl shows up just in time to save him. Now Carl has Mark's trust. With a bit of smooth-talking, Carl can get a reward or a favor from Mark--one that would make him more money than simply mugging Mark would have.
THE RAINMAKER
This one requires some charisma. Carl claims he can make it rain for Mark's crops (or that his medicine can cure Mark's disease, or that he can change the outcome of a sporting event in Mark's favor, etc). Mark pays up front, and if it actually rains, Mark believes Carl did it. If it doesn't, Carl convinces Mark he needs more time and/or money.
THE INVERTED PYRAMID
Like Rainmaker, but more about math than charisma. Carl sends out a free tip on some sporting event (say the first game of the NFL playoffs) to many marks. Half of them are told the Chargers will win, the other half, the 49ers. Whatever the outcome, half of Carl's tips will have been right.
The second week of the season, he sends out another tip, but only to those marks who received the winning tip from the week before. Again, half the tips say Team A, half say Team B, and in the end half of them will have been proven right.
He does this each week, until the day before the Super Bowl when he has a very small group of people who have received apparently perfect winning tips for the entire season. That's when he sells the final tip--who will win the Super Bowl--for $1,000 each.
The key to a good con is charisma and legitimacy. Maybe you imagine Carl as a sleazy, underhanded crook--easy to spot because he feels like a liar.
Carl?
But for a con game to work, Mark has to trust Carl completely (con is short for confidence, after all). That means Carl is going to be the friendliest, most humble person Mark ever met.
Carl!
Man I can't wait to write that character.
Anyway, what have you been researching lately?