Let me explain more.
You can put your own crystal and metal INTO the market for other PLAYERS, to buy at the price you set. And since your merchandise is in the market you can help determine the price a little bit.
Also if you sell a lot of (Crystal/Metal) to the MARKET, the price will go down.
And if you Buy a lot of (Crystal/Metal) FROM the MARKET, the price will go up.
** TIP **
When buying or selling large amounts of Metal/Crystal, Pause the game (Pause Break Button on keyboard) and buy it, the price will raise or drop quickly.