Ditto to what John and Marcia said. Since last year was my first time, I felt like I was a part of something much bigger than myself and my small group of ceramic buds....definitely broadened my definition of the words "potter" and "ceramics" and gave me such an appreciation of how hard people work in this field and how grateful most of them are to be doing what they do. There was so much passion for clay......for art.....for each other.
I do use a vent, L&L downward draft coming out of the bottom. Because we built the shop and then put the kiln/vent in, we were able to put the vent where we wanted it. So, a hole was cut through the side of the shop and the vent mounted and everything around was sealed up. Like Neil said, the heat is welcome part of the year. And in the warmer months, I open windows, doors, and run a fan. I have always used a vent, so I really don't have a comparison. Looking at dhPotter's pictures though, that tells me that there would be a difference!
It really depends on your farmers Market. I live close to a major ski area/summer destination. That farmer's Market is AMAZING! New crowd every Saturday. So there is always a new opportunity for sales. In my town, the farmer's market is small. My town is small. The crowds are there for food and consumables mostly. I don't do this one, I am concerned about oversaturation in my small burg. But it could be a great event where you are!