When pundits discuss "free trade," most people think of international trade, eliminating tariffs, import quotas, and the like. That's because the Constitution's Commerce Clause -- the one Congress has been using and abusing for decades -- grants the government the power to "make regular" trade between the several states. For example, Oklahoma can't ban imports of beef from coming across the Red River and New York can't have a different licensing regime for long-haul tracks entering from New Jersey rather than Pennsylvania.
While this commerce- (and liberty-) enhancing feature of our federal system has required a Supreme Court reminder for traditional wine retailers in recent years, Americans have generally taken for granted that buying and selling products between American jurisdictions is perfectly normal.
It may surprise you to learn, then, that in Lake Elmo, Minnesota, proprietors of a 40-year-old family farm that yields flowers, pumpkins and Christmas trees, are facing fines and 90-day jail sentences for attempting to sell their products in that town. The reason? Part of their farm lies outside city limits, and in Lake Elmo it's illegal for farmers to sell products -- even from their own land -- unless they were grown within the city. You can view a short video about their story here:
Thankfully, our friends at the Institute for Justice are stepping up to defend these folks for making a living by engaging in domestic free trade. This blatant protectionism is harmful and foolish when practiced with foreign trading partners, and is all the more repugnant when practiced against one's own neighbors who provide the community with valuable goods and services. That these law-abiding entrepreneurs face potential jail time for the crime of "selling produce across city lines" is anathema to the Constitution.
You can read Cato's work on agricultural free trade here