As I recall it, after creating a huge butter pat market from nothing, one day the aliens just walked away from it. Probably some deep political philosophy behind it that I missed when I first read it 30-40 years ago.
The Big Pat Boom by Damon Knight is about aliens creating a market out of cow dung patties just for fun. It's pretty hilarious how they make distinctions like "ooh, an Emperor pat! With a double whorl!" The humans are sucked into it and prices rise, a typical bubble.
Then the aliens just leave and the pats aren't worth...uh, you know what I mean.