Black market

What exactly is the "Black Market"? It isn't a certain place, or a myth but more of a system. A system where something as casual as gum is bought without tax. Most of us will understand how vast this market can actually be. From a pack of gum to illegal weapons, even on some black markets, nuclear missiles. Another thing on the market for sale are "services". These can be from plumbing to prostitution or even assassinations. So in some aspects this market isn't all bad, but not all is good either. We have all had some encounter with the Black Market if we know it or not. What do you think should be done with the Black market? Maybe make a certain place for it and police it or leave it be?