3.4  Imports  Commodities can also be obtained through imports, in which case they're not restricted by the limits of supply. Any commodity may be imported, but each company may have only one import line at a time. Each import has an initial setup cost, but after this all imports are made at the original start price for the commodity (whatever the current market price).
The setup cost for an import is equal to the size of the import times the original buy price for the commodity. This is the same as it costs for the commodities imported each turn. In a turn when you order a new import you pay both the setup cost and the cost of the actual commodities imported.
Once an import order has been made the import continues every turn until a new import order is made (that is, you continue to receive the commodities, and continue to pay for them, every turn until you decide to change it). Any new import order you make cancels the previous order.
When you change an import you recover half the setting-up cost of the old import order (this applies whether or not the new import is larger or smaller, or zero, or a different commodity).
Note: Imports are expensive to set up, but after that they're often cheaper than buying from the market and more reliable than trading with other companies. To cancel an import, replace it with a new order, making an import order of zero (of anything). When you cancel an import order you can actually liberate some cash (this can be very useful when you're trying to rescue a company that someone has mismanaged).

3.5  Buying & Selling Commodities  Each company can buy and sell commodities by setting a desired stock level (see 6.22 for the STOCK action), a price premium (for buys, see 6.19 for the PREMIUM action) and a price discount (for sales, see 6.7 for the DISCOUNT action). You can also suspend trading in a given commodity (using the HOLD action, see 6.14). All these instructions remain in force until you change them.
In the buying and selling phase if the stock held by the company is less than the level you've ordered then the company attempts to buy. If the stock held is more than the level ordered then the company tries to sell. When the company is buying the price offered is the current price for the commodity PLUS the price premium. When the company is selling the price offered is the current price for the commodity LESS the price discount.
Buys and sells are resolved by matching the buys and sells ordered by all the companies, plus the buys due to consumer demand (see 3.6). Where the totals of all the buys and all the sells don't match then priority is given to the best prices (ie. the biggest premiums or the biggest discounts). Where different companies are offering the same price then the priority is company order (see 3.9).
All buy and sell transactions are made at the price offered. Buy prices are always greater than or equal to sell prices (they can be the same, but if they're different then the buy price is higher) and the difference is how the commodities broker (see 5.1) makes a profit.
Note: The number offered for sale and the number wanted for purchase will rarely be the same, so there's almost always someone who doesn't get what they want. If you really want to buy something, then offer a price premium. If you pay more then  you're more likely to get the goods. If you're really keen to sell, then offer a discount. The lowest price will make the sale.
Note: Each company can offer only one price for buying and one for selling. If you want to offer the same stuff at two different prices then you need to put them on sale in different companies (and use give & get actions to deliver your stock to the right company).
The Price Table (see reference section) shows the initial prices for each commodity, and the commodities market report (part of your game report) shows the current prices for each turn.