Hello Mr. GreenBean,
If I were exploring a Farmer's Market, and I came upon a vendor who was selling roasted coffee, I wouldn't expect to see him actually roasting the coffee on site. And if he were to be roasting, I would question the quality of the end product, because of all of the factors that go into roasting the coffee and all of the things that can go wrong. Unless you're in an enclosed location, the temperature, humidity, wind, and especially dust and dirt flying around the market would be a big turn off for me.
Instead, I would look for a clean, nicely displayed booth that has tables full of rows or bins of half-pound or one-pound bags of freshly roasted coffee (roast date on the bag.) I would prefer to buy half-pound bags of several kinds of roasted coffee, instead of a full pound of something that I may or may not like. That way, I could go home and try several kinds of coffee, and then go back the next week and buy more.
I know someone who used to sell roasted coffee at Farmer's Markets - along with cups of brewed coffee. It got to the point where packing up (and breaking down) the setup for the brewed coffee and keeping an inventory of cups, sweeteners, milk, etc. to go along with it wasn't working for him. It was just too much work for a small amount of profit. He eventually switched his game-plan, and just had a booth displaying bags of roasted coffee (roast date on the bag) and he did quite well for many summer seasons - until the Farmer's market went out of business, and that was the end of that.
Rose