A market is a concretion of a fair. A fair, where people gather to buy and sell, is commerce stripped to the barest essentials. Fairs happen regularly; markets are generally open on a continuous basis. Structures- financial, social, and political as well as physical- are built to improve the function of the market.
Collecting and reusing the wealth from all this commerce is the primary task of the market, and the most direct way to do this is to charge admission, and, incidentally, provide some protection from the world outside to those admitted. The medieval city, with burghers who also don battle gear or stand watch, and nobility competing with the commercial classes in the market, provides a vibrant image of a market.
Even in the earliest times we can see the distinction between the market of the hinterland, which supplies the city, and the international market for goods that are bought and sold over vast distances. The interior market of the city can function somewhat independently of the international market and vice versa. With the development of the nation-state came the notion of interior markets for nations, and this notion is of particular import for Americans as for over a hundred years the US has had the largest interior economy of the world.
The US has the world's largest interior market and it's not hard to see why- we've built highways and airports, and we send our children to school where they become proficient and highly productive of modern goods and services. For over 200 years we've been investing heavily in productive capacity, and any sober review would award the government with credit for many of those improvements. In short, the people of the US have a legitimate claim to some say in how this market is run.
Of course, this is hardly the only original thought we'll encounter on our journey to self-government. To be honest, many of us have a depth of historical knowledge that would fit comfortably in a few Classic Comics books. On the plus side, it can be a lot more fun learning about this stuff when you're old enough to reach the top shelf. Recommended reading- The Disastrous 14th Century, by Barbara Tuchman .