black market

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The illegal business of buying or selling currency or goods banned by a government or subject to governmental control, such as price controls or rationing.
• noun A place where such illegal business is carried on.

from the GNU version of the Collaborative International Dictionary of English.

• noun the illicit buying and selling of goods, in violation of price controls, rationing, tax laws, prohibition of sale, etc.
• noun a place where such illegal commerce is conducted, or the entire system of such illicit commerce considered as a whole.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun trade that is in violation of restrictions, rationing or price controls
• noun the people who engage in such trade (considered as a group), or that sector of the economy

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun people who engage in illicit trade
• verb deal in illegally, such as arms or liquor
• noun an illegal market in which goods or currencies are bought and sold in violation of rationing or controls