black market

Từ điển Oxford Advanced Learner 8th

ˌblack ˈmarket [black market black markets] BrE NAmE noun usually singular
an illegal form of trade in which foreign money, or goods that are difficult to obtain, are bought and sold
•to buy or sell goods on the black market
•a flourishing black market in foreign currency

Example Bank:
•During the war, there was a thriving black market in food.
•You could buy anything you needed on the black market.