In MMOGs, the ability to buy and sell items between players (and I'm talking about transactions with in-game currency, not real-world gold farming) has created actual markets. Prices paid for items rise and fall with demand and scarcity. You might make a big investment in one kind of item or material that has a high price only to find that the price has collapsed because a hundred other players started investing in the same thing. Fads and trends mean that even long-established items may experience dramatic price changes and create market opportunities for canny players. In some games, such as EVE Online and my own Pirates of the Burning Sea, you may even lose your investment outright due to your ship being destroyed or seized by enemy players.
Such economic gameplay is both broad and deep. It lets players participate in a variety of ways, from being simple consumers to masters of enormous trading companies. And throughout is the most important sink of all: risk.
Normal sinks like item purchases and even consumable item replacements have no real risk involved. You just repeat the same transaction with predictable results. But in a market, risk is very real and can drain money away from you in surprising ways.
Fable II tried to do this with markets. You could buy low and sell high, mostly by exploiting sales and price differentials between towns. The faucets were turned on so high, however, that the income you might make from these simple markets was worthless.
Let's construct a simple market feature for Assassin's Creed II, since that's the game I'm currently obsessing over. Our goal is to give the player with an excess of money the opportunity to risk it in return for possible benefits.
At your stronghold villa there is a loyal servant of your family known as the Trade Master. When in your villa, you can ask him to invest money for you in one of several commodities. You also choose which trade route to invest in: Florence-Tuscany, Tuscany-Venice, and Venice-Florence. (Each trade route option is bidirectional, to prevent the options from multiplying.)
Having made an investment, it will come to term after thirty minutes of gameplay. Investments have a minimum amount of $10,000, and you can have multiple investments going at the same time.
The game rolls dice behind the scenes to decide whether commodity prices for a given trade route go up or down for a given period of time. Usually you earn +/- 5%, occasionally you earn +/- 15%, and very rarely, you earn +/- 30%.