"curb market" - a regional term meaning "An informal market formed by parking vehicles containing goods to be sold at a street curb." Some such markets, with increased popularity over time, gained permanent homes inside large, open buildings.
Thank you Chris.
That was my initial suspicion, which prompted the query.
I could not understand how a letter could be addressed to a Curb Market, even if it was in a building, believing there would not be a reliable address for any of the vendors there.
It was certainly curious.
Here in Western Australia we call them "swap meets"
and usually held in car parks.