Slave trade

The Sixth Doctor considered the slave trade to be the "dark heart" of England in 1756 and one of Earth's worst abominations. (AUDIO: The Behemoth)
By 2011, a black market slave trade smuggled aliens, such as Skullions, across Earth. (TV: The Man Who Never Was)
In the 22nd century, Harry Sline was arrested for slave trading between Mars and Venus. (AUDIO: The Curse of the Daleks)