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There's a disagreement in this thread: mmorpg.com/discussion2.cfm/thread/182960
1. A free market economy where you set your own prices and sell things directly to other players. You might have tools such as player vendors or an auction house to assist you in finding customers, but you only sell things when someone wants to buy them. You attempt to find a niche in the market and sell items and services needed by other players. Supply and demand determines the value of things, and it's possible to make money speculating on prices. The game would be designed with interdependence in mind to encourage trade. On the downside, it's possible to be scammed or lose money on a bad investment. There are ups and downs in the economy and price fluctuations which can either work to your benefit or detriment.
2. A planned economy where the devs set the prices before the game launches based on predictions about the time and effort it will take to make various items. You sell you items to NPC merchants at the given price regardless of supply and demand. Other players can then buy those items, again at a set price. The game would be designed to let everyone make money doing what they enjoy without risking anything or having to interact with anyone. Your income is constant. There is no way to fail or directly lose money, but no way to win big either.
Apologies if I didn't do justice to one or the other in my descriptions of them, but hopefully that gives you the general idea.
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