Here in New York City, I'd have to agree with Michael... Farmer's Markets aren't great venues. Subways are the best, downside, very competitive for good spots and it does huge damage to your hearing, Parks... some good some bad, have to find the right place, one place in Central Park is a real money maker for me, others really suck. I can play all day at the Union Sq. farmers market, get lots of people stoping to listen, NO one giving a ha'penny. Not sure why, but I have never seen a busker make money at a farmer's market in New York City. It may be better out in the country, hope so, best of luck.
Cheers
Larry