A black market was a market consisting of all commerce on which applicable taxes and/or regulations of trade were being avoided. Items could be purchased on the black market by those not legally able to purchase them otherwise, often for a substantial fee.
This indicates that Quark likely was still involved in the black market after the Occupation had ended. The character of Rulod, who Quark identifies as controlling the black market on the station, was developed in early drafts of "Emissary". (The Making of Star Trek: Deep Space Nine, p. 93)