In the 1970s, Forrest Ackerman sold this story to Vertex for $100:
Cosmic Report Card: Earth
He resold it four times for the same amount, and it’s been translated into three languages.
The “shortest horror story ever written” is usually attributed to Fredric Brown:
The last man on Earth sat alone in a room. There was a knock on the door.
Ron Smith shortened this further by changing knock to lock.