Garage Sales:

Every year on May 10th our town has a Community Garage Sale. This is an event where members of the community collude in a non verbal agreement that goes -

If you take my junk and store
it in your basement, I’ll come
and get your junk and store
it in my basement.

Because this is obviously insane, an exchange of cash is mandatory in order to lend this practise a flake of legitimacy.