Complex sentence: A common market is the first stage towards the creation of a single market, and built upon a free trade area with relatively free movement of capital and of services, but not so advanced in reduction of the rest of the trade barriers.
Split into simpler sentences: A common market is usually referred to as the first stage towards the creation of a single market. It usually is built upon a free trade area with relatively free movement of capital and of services, but not so advanced in reduction of the rest of the trade barriers.