BUY A CAT IN THE SACK
to buy an article without first inspecting it ...c1380 obs.
Compare obsolete French expression acheter chat en poche, or en sac, literally to buy cat in poke, or in sack.
Also in various others languages, as found here.
FIRST DOCUMENTED USE
c1380 - See example below
"...To bye a catte in þo sakke is bot litel charge;
to bye chirchis by symonye semes sumwhat siker:
bot for to bye þus heven and broþerhed of Crist,
hit semes chaffere of Lucifer, and withouten grounde.
To buy a cat in the sack is but little charge;
to buy churches by simony seems somewhat safe;
but as for buying thus heaven and brotherhood of Christ,
it seems chaffer [= trade] of Lucifer, and without ground.
From: Select English Works
By: John Wyclif, c1380
• A New English Dictionary on Historical Principles,
James Murray, 1887-1933