"buy a pig in a poke"

The Pig?
Creative Commons Attribution Share AlikeThe Pig? - Credit: AJ Ashton
To buy something inferior without fully examining it first.

Variants of this phrase can be found far beyond Scotland.  It derives from a con trick, whereby an unsuspecting customer bought what they thought was a pig in a sealed bag ("poke").  On opening it, they would discover they had actually purchased a less valuable cat or dog.

The French equivalent is acheter un chat en poche (to buy a cat in a bag).

This same con trick is also the origin of another well-known expression: to let the cat out of the bag.