Magratheans are the inhabitants of the legendary planet, Magrathea. Because their planet-building operation was so successful, they reduced the rest of the galaxy to abject poverty.
Slartibartfast as seen in the 1981 TV series.
Since no planet was rich enough to buy a planet from them, all Magratheans decided to sleep through the recession until the galactic economy returned to a level which could require and pay for their services. The only individual Magrathean to appear in the series is Slartibartfast.