pig (in) poke

Translations
Кот в мешке
Buying a used car without checking its condition is like buying a pig in a poke.
Покупка подержанной машины, не проверяя её состояние, - это как купить кота в мешке.
Definitions
A 'pig in a poke' refers to something that is bought or accepted without first being examined or properly evaluated, often leading to an unpleasant surprise. The phrase originates from a time when a piglet (pig) was sold in a sack (poke), and buyers who didn't check the contents might end up with something less valuable, like a cat, instead.