Black market

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.

There was a black market in alien species on Earth in the 21st century. John Harrison payed millions for a group of Skullion slaves that had crash landed in Central Asia. They were brought across Europe and then across the English Channel. (SJA: The Man Who Never Was)