The black market allows players to buy and sell Metal and Crystal in exchange for Credits. The purchase rate for these resources fluctuates over time, and is also heavily influenced by players purchasing and selling a particular resource. It works on the basics of supply and demand. If lots of players are buying something, e.g crystal, the prices of crystal will go up dramatically. If players are selling lots of something, e.g metal, the price will go down until it reaches a set price. At the start of the game, you begin with plentiful metal but little crystal. As a result, many players will sell metal and buy crystal at the very start of the game, before prices can fluctuate.
When you sell resources directly to the black market, you get 50% of what the black market is selling them for. However, if you're willing to wait for another player to buy your resources, you can
put them on the market for up to 100% of the black market's selling
price. However, you'll have to wait for someone to actually buy your resources before you get paid for them. The buyer still pays the full black market price, even if you were charging less. If two players are selling resources on the black market, the player who is charging less will get the sale. There's usually more demand than supply of resources,
so it's easy to get away with selling at 100% of the black market rate.
The practice of buying a resource low to later sell at a higher price is theoretically possible, but not practical in a game where war is always around the corner.
The graph shown in the centre of the black market screen shows both prices for crystal and metal over a recent period, with metal represented by a yellow line and crystal shown with a purple line. If one was to take a 'buy low, sell high' approach to the black market then this would be helpful as a quick heads up on the relative prices of both resources.