There are two sides to trade in Guild Wars: the game side and the player side. The game side is simple: NPC merchants sell their wares at prices that fluctuate a bit due to supply and demand. If the economy goes completely crazy for one item (think Spiritwood Planks when the Black Moa Chick was introduced), prices will jump, but the merchants will simply "run out" of that item before the cost has a chance to get really out of control. If demand is very low, prices for both buying and selling will drop to an established minimum and stay there until the economy dictates otherwise. The system is set up so everything stays at a reasonable rate.