Is it illegal to sell mushrooms at farmers markets?
Many vendors around the country sell wild harvested as well as cultivated mushrooms at farmers markets. However, some local or state health departments require a license to sell mushrooms. Last year in Iowa City, Iowa, vendors selling wild-harvested morel mushrooms were required to get a "potentially hazardous food" license before selling at the local markets. I don't think bartering will change the need for a license if one is required.
This website Licensing at Farmers Markets has information regarding licensing for selling food at farmers markets in Michigan. The website has a fact sheet, "Temporary Food Establishment Operations Checklist," that will be helpful.