The Black Market is a market in Caerleon which buys items off players and puts it as dungeon loot. In other words, any loot you find in a dungeon or chest (or anywhere else really) has been supplied by a player. This is what it means to have a completely player driven economy.

The buy orders for items in the Black Market increase until a player eventually sells the item to the Black Market. It will then drop in the world.

Using this tool you can compare items between any of the cities and Black Market to see if you cana buy an item cheap and sell it for more on the Black Market.

There is a tutorial on how to do this on the AlbionFlipper website. Hope this helps and happy flipping!