The core of the game revolves around selling resources and products you create to the cities for a profit. Not only must you produce these products efficiently by mining high value resources or getting components cheap you must also sell the end product at a high price.
Every product in every city has a statistic called the "Demand". This represents how much above or below the normal price that product is. Normally the demand is at 100%, if the demand is 200% then it is valued at twice as much as normal. The average demand of all products in a city determines whether it is growing or declining and at what rate. Average demand of over 200% will cause decline.
The actual amount a product sells for depends on what "tier" it is in. Products gained directly from resources (Wood, Crops, Metal etc.) are considered tier 1 products and sell for the same amount as the demand. Components products (Alloys, Polymers etc.) are considered tier 2 and sell for 2.5 times their demand value. End Products and Services sell for 5 times what their demand value is.
The "Volume" represents how much quantity of that product the city will accept in a single turn. It also represents how many units of that product it will take to lower or raise the demand by 1. The volume applies to ALL sells of that product happening on that turn, including other players’ ship or structures. If more units are being sold than the volume amount in a single turn, the actual amount sold will be divided evenly, by percentage, among all the sellers. That means that a ship trying to sell 10,000 units will get more through than another ship that is also trying to sell 500 units at a ratio of 20 to 1 (500 is 1/20th of 10,000).
Ships and structures that cannot sell everything they have been ordered till will sell what they can and try to sell again next turn. However, if a unit is unable to sell a single unit of product which it has been ordered to it will cancel its current order and move to the next one.
Buying works in an identical way as selling, however, buying products will cost +100% (double) more than the listed value and will raise demand. Buying a product valued at 400 will cost 800 per unit for example.
The Demand of all products at all cities slowly rise naturally at a rate of about 100 per week (0.1% per turn over approximately 1,000) turns. When the population of a city is low, below 5 million, the higher tier product categories demand rate slows to 1/10th of normal. When a city exceeds population of 2 million, 3 million and 5 million the demand for Tier 2 (Components), Tier 3 (End products), Tier 4 (Services) products respectively begin to rise normally. This makes it easier to supply new cities with simpler products while they are starting out.
In order to trade with another corporation you must agree on a contract which formally declares how many credits are being traded for what product. When giving an order to transfer product to another corporation you must declare the agreed upon contract to use or there is nothing enforcing the other corporation to pay. Contracts are also used for repeating trades with other corporations which maybe steadily supplying you a missing component for one of your factories on a regular basis. These contracts can be specified to deliver a certain amount of product to a certain location and for a price that is either static or scaled against the value of that product at a nearby city (i.e. 10% lower than the current price in Manila).