Trading in Elite Dangerous occurs in systems with commodities markets, which usually exist in stations. Each system will have a single or mix of market-types, e.g. industrial, agricultural and so on. Some stations also have rare commododies, which are only availible at that station and have a greater value the further away you travel.
If you have stolen/scooped cargo (e.g. from a destroyed ship) you cannot sell this on the normal commodities markets in stations. Instead it must be sold on the black market.
The prices in the Black Market are lower than the open market.
The galactic market of Elite Dangerous is modelled by a back-end server running a trading simulation that takes into account various factors such as local resources and system population to model the supply and demand. This simulation will also include AI players that will be trading between systems affecting the prices in real-time, as well as actions taken by human players.
As the markets are no-longer isolated as they were in previous games in the series, it is now possible that profitable trade routes may become unprofitable over time if there is sufficient action from other players that shifts the market. For example if there was a strong demand for a rare (and profitable!) trade commodity at a station that led to large numbers of players trading that commodity there, eventually the profits would decrease as the demand would be sated by the large supply of the commodity.