pig in a poke
An object offered in a manner that conceals its true value, especially its lack of value. For example, Eric believes that buying a used car is buying a pig in a poke. This expression alludes to the practice of substituting a worthless object, such as a cat, for the costly suckling pig a customer has bought and wrapping it in a poke, or sack. It dates from a time when buyers of groceries relied on a weekly farmers' market and, unless they were cautious enough to check the poke's contents, would not discover the skullduggery until they got home. The word poke dates from the 13th century but is now used mainly in the southern United States. The idiom was first recorded in John Heywood's 1562 collection of proverbs. Also see let the cat out of the bag.