Slave trade
From Tardis Wiki, the free Doctor Who reference
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)