Die Katze im Sack kaufen

Literally: To buy the cat in the sack

Example: “Kauf kein Auto über das Internet, sonst kaufst du die Katze im Sack.” — Don't buy a car online, or you'll be buying a pig in a poke.

English equivalent: To buy a pig in a poke

Origin: Market cheats are said to have put a worthless cat in a sack in place of the piglet, rabbit or hare the buyer had agreed to, and a buyer who did not look inside was fooled. The trick is told as a comic tale in the Till Eulenspiegel chapbook of 1515, where a cat is sold as a hare, and the short form etwas im Sack kaufen (to buy something in the sack) is already in the tales of the poet known as the Stricker, after 1240. Spanish has the related dar gato por liebre (to give cat for hare).