Black market

A black market was a market in which all applicable taxes and/or regulations were avoided. Items could be bought on the black market by those not legally able to purchase them otherwise, often for a substantial fee.

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