A car boot sale is a peculiarly British event where people gather up unwanted items from their house, drive to a field very early on a sunday morning, and sell stuff from the “trunk” (sorry, i’m English, I find it hard to use that term!) to other people.
A bizarre phenomenon
Alternatively, it’s like having a yard sale where the people buying stuff don’t know where ya live. All you do is pay a small sum to the organisers, who’re often local charities, and with 200+ cars doing the same thing it’s a great way to help fix the church roof and get rid of all the crud you no longer want.
Best places to pick up garden plants, household tools, steel-capped boots and old electrical bits, so the standard British garden shed dwelling absent-minded boffin inventor types will invariably head straight from church to the car boot on a Sunday. It’s traditional.