Ray… “Market cars” were used primarily in agricultural regions to transport farmers’ produce to market. Think of it as a farmer’s market on wheels; or at least the truck which carried the farmer and his goods to the market. Generally, a single farmer wouldn’t have enough produce to fill an entire car, but enough farmers along the way to the “big city” could.
I hadn’t heard of them either, but as I dig deeper into eastern narrow gauge history, I’m discovering that it was surprisingly common for a railroad to have at least one or two such cars on the roster. In many cases, they were listed as box cars, so unless they showed up specifically in a photo, one would never know they existed.