As the manager of a farmers market............check with the manager before lugging in an amplifier...........they don't really fit the scene in our county, anyway. Be consistent, and dress the part. Most managers strive to have high quality produce, so the musicians should follow suit. I'm talkin' basics: jeans are fine, if clean with no holes. Wear a shirt. brush your hair (or polish your head) Show up on time, and PLAY. Particularly if the market is putting you into the advertising budget, give them good worth. Have your business cards with you, as well as paper and a writing utensil. Assume that you will need to bring something to sit on, and your own shade. Not everyone will have a market umbrella for your instruments.
Have fun! Markets are a great way to learn how to perform.....its how we learned!