Violin Scam

A worthless item is sold as being of much higher value than it actually has. The key is to convince the mark he has information about the item that the seller doesn't.

Like this: One of a pair of con artists, posing as a retired violinist, leaves his precious violin as collateral against some small debt while he goes and fetches the cash. While he's gone, the other con man presents himself to the mark as an instrument dealer—business card and everything. He wants to have a look at the violin he saw in passing, and as soon as it's produced, he's delighted—as he thought, it's an original Stradivarius! It's worth hundreds of thousands of dollars! He must have it, but he has a plane to catch and can't wait for the old man to return—Dear sir, would you please give him my business card?

Now the mark's greed comes in. If he's a good man, he gives the old violinist his instrument and informs him of his good fortune, and the scam has failed (the cons have lost nothing, however). If he's a lowlife, however, the scam may succeed. He offers the old man a thousand dollars for his violin—the old man clearly has no idea of its actual value, but loves it like a family member. The price goes up and up, and eventually the con artist gives in, selling his beloved violin for ten thousand dollars. He walks out with the money, and the new owner waits a bit, then calls the number on the card. It's a false number, of course, and any professional will immediately recognize that the violin is worth perhaps fifty dollars, and the old man and the "dealer" meet up to split the take and get another cheap violin.

The expressions "It's a fiddle" or "fiddling" to denote cheating are said to derive from this venerable grift.

A variant is the "lost ring" scam, in which a customer approaches the mark (in this case an employee or manager) with a ring he claims to have found. The con man's accomplice will then phone the store, anxiously claiming the ring and offering to come directly to the store with a huge reward for its return—but the good Samaritan who found the ring can't stay to meet him. The con relies upon the employee choosing what looks like a practical solution: offering the finder a small percentage of the promised reward, perhaps straight out of the till. He isn't worried; after all, he expects to receive a much larger sum once the owner returns to claim the ring. Needless to say the ring is worthless and the "real owner"—and his money—never appear.

Compare Pig in a Poke.

Examples of Violin Scam include:

Film

In a flashback in Zombieland, the girls pull a violin scam with a ring. It goes something like this: At a gas station, Emma Stone is rich and pretty and obviously searching for a lost item on the ground. When the station clerk approaches, Emma explains that she lost her engagement ring, but she's got a plane to catch. She offers to reward him handsomely if he finds it and ships it to her. Later, he sees Abigail Breslin "finding" the ring on the ground. He gives her all the money in his cash register in exchange for the ring. Afterwards, Emma meets up with Abigail and it is revealed that they have a bag full of cheap rings just like it.
Done in an early scene in the movie Shade. Woman loses ring, promises gas station attendant $1000 to anyone who finds it. Bum finds ring, gas station attendant buys it from bum for $300, bum goes around a few corners and gets into car of woman...
A variant using a "winning" lottery ticket (of course a faked one) is used in the movie Matchstick Men.

Literature

American Gods features a detailed description by Mr. Wednesday. It's a plot point later.
In Going Postal, this is mentioned (as part of his Backstory, of course) as one of Moist von Lipwig's ways of keeping in practice.
In the Kingkiller Chronicle novel The Wise Man's Fear, Denna's foolish boyfriend Geoffrey falls for a variant known as "the weeping widow". In this variant, a woman stands weeping outside a pawnshop and when questioned by an apparent Good Samaritan claims that the man who was helping her sell her valuable wedding ring pawned it for a fraction of its value and ran off with the money. The woman and the "Good Samaritan" agree to meet the following day at noon to get the ring out of hock and sell it together. Usually, the "Good Samaritan" buys the ring on his own before the woman comes back, and the woman and her accomplice the pawnbroker spilt the money. Geoffrey, however, was one of those rare men who show up right on time. Denna, who has run this con herself, finds it dishonorable of her fellow conwoman to keep a decent guy's money.
Holly Black's Curse Workers: the White Cat has a variation of this involving a white cat. Cassel needs to get a cat out of a shelter. He's under 18 and can't just adopt it, so his friend comes into the shelter claiming her expensive white cat is lost and offering a huge reward. Cassel then goes in and claims he has the missing cat, but will need a white cat for his little sister as a replacement. The shelter worker, thinking of the huge reward, is willing to skip the background check and give Cassel the cat, expecting Cassel to return later with the "missing" cat.

Live-Action TV

Hustle has featured a variation on this known as "dog in a bar", where a supposedly valuable dog stands in for the violin. The song "Can't Con An Honest John" by The Streets details the same dog-in-a-bar variation.
Leverage used a variation with horses - although they had to actually steal a good one to demonstrate the speed. Then they just swapped it out for a similar looking but much less valuable animal and trusted the man wouldn't notice. And of course the ultimate object of the con wasn't to steal his money, but have him accused of insurance fraud.
- In "The Studio Job", the team refers to the con as a fiddle game, which confuses Parker and she keeps asking where the fiddle comes in. When she finally understands, she says "Oh, Eliot's the fiddle!". Later in the season, in "The Underground Job", they discuss running a fiddle game again.

Parker: Is Eliot gonna be the fiddle again?
Nate and Sophie: No.
Parker: Can I be the fiddle?
Nate and Sophie: No.

A variant was done in Only Fools and Horses episode "Cash and Curry" where the conmen were of Indian descent and the supposedly valuable object was a statue of a Hindu god. The con was played with a variation, where the two conmen claimed that they were unable to talk to each other because they were of different castes; and of course, the episode ended with a thoroughly Anvilicious scene where the conmen pointed out that only a prejudiced person attempting to swindle clueless immigrants would fall for such a scam.
One episode of Lovejoy was spent modifying a motorbike to convince a collector that it was the one T.E. Lawrence had been riding when he died. Varied chiefly in that the "authenticating expert" role was left to The Mark himself to play: all the "evidence" was circumstantial and never mentioned explicitly by the cons, and in some cases (a bent handlebar, a rattle in the gas tank), they even tried to hide it for fear of devaluing the bike - which, in a smaller variation, was not completely worthless on its own merit, just not worth quite so much.
- An earlier Lovejoy episode featured a Greek antiques dealer(played by BRIAN BLESSED!) who conned a Japanese business man into purchasing a Satsuma tea-bowl for millions. The tea-bowl was authentic, but the provenance stating that the bowl was once owned by Emperor Hirohito and that the Emperor had drunk from it just before signing the Armistice that ended World War II was forged. Lovejoy and the businessman(along with a wealthy and attractive American widow whom BRIAN BLESSED had scammed earlier) got back at him with an elaborate scam involving a forgery of a Russian religious icon(a market The Mark was attempting to corner) that the businessman had in his possession, complete with an authenticating expert, and a trick where Lovejoy hand-sewed two bags, one for the fake icon and one for the real icon, from antique velvet spritzed with incense(so it would smell like it had been left inside a Russian church for decades) and sprinkled the interiors of the bags with paint chips from destroyed antique paintings so they could provide The Mark with all the "paint samples" he would require to authenticate the icon.
In Another Midnight Run, a Made for TV Movie based on the original Midnight Run film as part of the Action Pack, a pair of grifters Jack is trying to bring in use the wedding ring variation to score some cash in order to buy a car when they & Jack are stranded in a small town. The male of the two narrates the ploy to Jack while she sets up the marks, telling them she lost her ring. He explains that the heart of the con is the greedy nature of the mark. "You can't cheat an honest man."

Oral Tradition

An old joke/urban legend about an antique dealer, a cat, a saucer, and a greedy customer involves this. The customer is shopping in a crowded antique store. He sees the store cat eating from a saucer, and recognizes the saucer as very valuable example of Ming porcelain. Assuming the dealer doesn't know what it is, he offers to buy the saucer for $5.00. The dealer declines. The next day, the customer comes back, and this time, offers to buy the cat, offering a much higher price. The dealer agrees, and as the customer is leaving with his new cat, he says "I expect he's used to eating out of that saucer. Let me take it as well." The dealer replies "Give you that saucer? Never! Do you have any idea how many cats I've sold with that saucer?"

Western Animation

An amusing variation happened in Top Cat: a pair of con artists trick an immigrant hot dog vender from the neighborhood into buying stocks in a floundering Nova Scotia oil company. T.C. then tricks the scammers into thinking he's a Texan millionaire. While they are meeting in his "office" (the alley, only spruced up), T.C. leaves for a moment only for the "teletype" (the output of which actually comes from Benny hiding under a table with a typewriter) to state that the well struck oil and the value of the shares skyrocketed. The scammers then rush to the hot dog vender and buy back the shares at triple the price. In other words T.C. managed to pull the violin scam for a worthless item the marks had previously owned!