noun
black marketeer; black market trader
Refers to a person who deals in illegal or rationed goods on the black market. Often associated with post-war or wartime contexts, but can be used for any underground trader.
戦後、多くの闇屋が物資を高値で売っていた。
After the war, many black marketeers sold goods at high prices.
彼は表向きは商人だが、実は闇屋だという噂だ。
He is ostensibly a merchant, but rumor has it he's actually a black market trader.
Compound of 闇 (yami, 'darkness; underground') and 屋 (ya, 'shop; dealer'). The term likely arose in the post-war period to describe those who traded on the black market.