Try out different varieties of vegetables
Farmers love to grow small-batch vegetables, like multi-colored carrots and potatoes with a much wider range of flavors than supermarkets carry -- sorry, Red Delicious. Some are even breeding their own varietals with their own cooking characteristics -- not something you can identify on-sight.
So what are you supposed to do with these unknown veggies? Farmers have seen their crop from a seedling to a full blown potato, so you best believe they know which spud is better for boiling and which is better for roasting, just ask.
Don’t buy out of season
When you’re buying from a farmers market, you're assuming the goods are in season and produced by sylvan elves and fertilized by whimsy... but some farmers might take shortcuts, like growing produce in greenhouses to imitate a natural growing season. A quick search for "local harvest calendar" and "your geographic region" should reveal what is and isn't in season, hopefully protecting your from farmer bamboozlement.