Tips for playing at farmers markets etc?

Yeah -- don't bother. It's early in the morning (well, at my local it is); people are there to get their shopping out of the way before their day starts, and they're not likely to stick around to listen to you.

But if you do play, don't worry about the vendors getting bored. Their onions and cabbages don't get any more interesting during the course of the day, either.

Cheers,
Michael