In the 1970s, Forrest Ackerman sold this story to Vertex for $100:

Cosmic Report Card: Earth
F

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.