There is no card called "Farmer's Market". It's "Farmers' Market". There are lots of farmers.
I was surprised when I did the research there, and think this may be a thing that is gradually changing in the language. When I researched Worker's Village, I went with Worker's over Workers' or Workers; it gets used all three ways. Without additional research now I bet Farmer's Market is the original term, and it was Farmer's because what you are talking about is a farmer's market. A market for farmers is a farmer's market. But people have gotten hung up on, wait there are multiple farmers, I don't want to look like some idiot who doesn't understand apostrophes. So over time it has switched to farmers' market. And some people dodge the issue with farmers market, or have just run out of apostrophes.
You can quickly see it all three ways with a GIS for "farmers market sign" without quotes.