Markets provide access to the Global Market. A single market in a city can buy, sell or ship, but only one at a time. How quickly a market processes an order is dependent on the number of workers in the market and the efficiency level of the market.
When a market buys from the Global Market, it pays the current DER (Daily Exchange Rate) for the resource purchased. Purchased resources go directly into the stores of the city. Regardless of purchase orders, a market can only buy what the empire can afford.
When selling to the Global Market, there is a 10% "storage fee" kept by the Global Market. Thus, food will always be purchased at 0.90 per unit, but the price paid for bricks, lumber, and metal will fluctuate according to current quantities.
The Daily Exchange Rate for a particular resource varies according to the quantity of resource that is in the Global Market. If there is a scarcity, the price goes up. If there is a glut, the price goes down. Food is the only exception, and is always priced at $1.
A market can also post resources from the city for sale. The player must specify the maximum quantity per resource he is willing to sell. The player must also specify a price per unit sold. If another player wishes to buy some or all of the resources, the Global Market will handle the transaction and shipping, charging 5% of the sale.