I'm getting the feeling that you guys are split on this, as I am.
Frankly, I'm against it. BUT, as was said, it's a friggin farmers market!!!!! Enjoy the autumn day playing music (albeit 4 times a song) and take the money and buy my girlfriend lunch.
Hey, OP. I saw that you're from Nanuet, and I thought I might have something to offer regarding the local farmer's market scene. Seriously? It is SO low pressure, that you do not need to get that worked up about having a small repertoire. Work on it if you can, but consider where you're playing. And if you're still worked up about it? Ask locally about that particular farmers market.
I was at a farmers market in Bergen County a few weeks back. They had an acoustic duo performing, and people seemed to be appreciating what they were doing. The thing is, THEY were only playing about 5 or 6 songs, and then they'd just run through their set again. I didn't see the farmers market organizers hounding them, saying "Why are you only playing the same 6 songs?!?!"
Truthfully? It was a little annoying to me as a musician, but again consider the context: it's a farmer's market. People who are there are much more concerned with buying fruit, vegetables, bread, etc. The musical program is the least of their concerns.