Pig in a Poke

You've just happened onto an absolute steal of a deal. Seriously, the guy apparently doesn't know what he has, and his asking price is far too low. So you buy it, sight unseen. Or it's simply something that you want, and the price is decent. Again, you buy it, sight unseen.

Then you unwrap it and take a closer look. And it's not what you thought you were buying. Maybe it doesn't work nearly as well as the seller said it would. Maybe it doesn't work at all. Maybe it isn't even close to what was described. You just bought a Pig in a Poke.

This trope doesn't just apply to any crooked bargain, though. A Pig In A Poke is something that is represented as one thing, purchased, and turns out to be something else.

This trope normally has two variations.

A flat out scam. The seller knows exactly what he's selling, and that it's not worth anywhere near the price he's asking for it. In this type, it's the buyer who gets fleeced.
An honest gamble in that neither side of the deal knows quite what they've got. In this type, it may be the seller who comes out on the short end of the deal.

Purchasing one may make you a Unwitting Pawn, if some evil organization is involved. A Pig In A Poke usually doesn't do any harm to the buyer, excepting of course for his wallet. For objects that actually do harm the purchaser, see Artifact of Doom and Artifact of Death.

The old Latin saying "Caveat Emptor" goes to show that this trope is Older Than Feudalism. Fables about fools purchasing worthless junk at a premium can be found around the world.

The Trope Namer is a Middle Ages confidence trick wherein a con artist would sell somebody what is supposed to be a suckling pig in a sack or poke. In fact the hidden meat would be a cat. The same con gives us the phrase "to let the cat out of the bag". Known in Spanish as "dar gato por liebre" (giving a cat instead of a hare) after the Medieval practice of selling cats instead of hares for food. Unlike the pig, hares and cats look quite similar when skinned.

A Pig In A Poke that specifically uses illegal items, resulting in the buyer having no way to involve the authorities without getting in trouble themselves, is a Beat Bag.

If the seller shows you what he's selling, but then uses sleight of hand so that isn't what you actually get, its Good for Bad.

Compare Violin Scam, where the buyer is convinced that he's scamming the seller by the seller's confederate.

Examples of Pig in a Poke include:

Film

The basic plot of Burn After Reading is that a few morons blackmail the CIA into buying one of these.

Literature

In 'Harry Potter, leprechaun's gold vanishes shortly after you pick it up, rendering it completely worthless except for screwing over people you owe money to (which is exactly what it gets used for).
Conversed in Neil Gaiman's American Gods: Wednesday is talking about some of his favorite grifts, one of which involves a violin, two grifters, and an upper-class waiter as The Mark.
In Roughing It, Mark Twain describes a common type I: combing an otherwise worthless mine for one tiny chunk of rock containing silver or gold, presenting it to the assay office as an "average" sample, then selling shares in the now grossly overvalued mine.
This is one of Moist von Lipwig's preferred grifts. He likes to set people up to think they're ripping him off. You can't fool an honest man, which is why Moist never tries.
One back-cover blurb for Andre Norton's Sargasso of Space actually described the trade contract for the planet Limbo as a "cosmic pig-in-a-poke." When the crew of the Free Trader Solar Queen "unwrapped" it, they found the government's Survey service had sold them trade rights to a planet that'd been heavily nuked at some time in the distant past. It was unlikely to have any inhabitants with whom they could make deals....

Live-Action TV

Haven has a variation in between Type 1 and 2. Duke presents a chef with a box containing an exotic ingredient and names a price. The chef then has to decide if he wants to buy the item without seeing it first. Duke knows what the item is worth but he is not really scamming the chef. They are old friends and it's a game they have been playing for years. Sometimes Duke will overprice the item and sometime he will underprice it, making sure that his friend will not feel taken advantage of.

Music

This comes up in the folk song "Quare Bungle Rye." Jack thinks he's getting good whiskey, but the seller slips him a baby in a basket instead and runs off.

Tabletop Games

Magic: The Gathering
- One can purchase old Arabian Nights boosters, albeit at a hefty price. However, from time to time, these packs contain as their "rare" one of the five most common cards in the game, a Mountain, in which case you just threw away about $200. Congratulations.
- The practice of auctioning repacks on eBay involves a large bundle of otherwise-unsellable cards, with a few high-value rares purportedly thrown in, which is then divided up into booster-sized repacks and sold off for up to a couple of dollars each. Needless to say, anyone buying these repacks is quite a trusting individual.

Video Games

Knights of the Old Republic II contains a type II example: a salvager tries to sell you a holocron for 500 credits. When you ask to see it, he says that it doesn't work that way: he doesn't know the item's real value, so you would both be gambling on this deal. (it turns out to be fake)

Western Animation

In South Park when the boys are involved in a tooth fairy scam, Butters buys what he was told were Chinese teeth, but are actually cat teeth.