Trading consoles are used to buy and sell the various commodities around the stations, moon outposts and planet based cities. These consoles are generally found near the Admin desk areas and allow you to buy and sell many different types of legal cargo. You can generally sell legal cargo to these areas in large quantities but you might have to slow down and allow some time between trades if selling extremely large quantities.
There are hidden trading consoles around the same areas that allow you to buy and sell illegal cargo such as alcohol, drugs, etc... but at a much smaller quantity and slower reset time.