The Nin homeworld was the home planet of the Nin, which was destroyed by solar flares. Looking for a new homeworld, they thought that they legally purchased the planet Earth, not knowing that the world was already home to another species and that the sale itself was done as the result of a galactic real estate agents' mobile device unintentionally being used as a gaming console by the human boy named Robbie.
After the Fliant Wormbleeder was contacted by the Doctor, he offered the Nin a new home planet, as well as new planets for the Ssard and Hoolox. (COMIC: The Day the Earth Was Sold, The King of Earth)