Market
From Universe Wars
First you must build a school. Second research Dimplomacy and Trade. Once that is completed build a Market and at least one Cargo Warehouse. Each Cargo Warehouse lets you buy and sell up to 10,000 units of any resource. It takes three ticks to clear the warehouse.
With the market, you can buy or sell Iron, Wood, Food, or Oil for an amount of gold. The value of each resource rises or drops at random, so you may have to wait 1 tick for the price to rise or 6 ticks for the price to rise, nobody knows. But remember to do this you need at least one Cargo Warehouse.
Keep in mind that your warehouses take 3 ticks to clear, which means that if you plan on selling 8 million iron and buying wood with the proceeds, you'll either need to use tokens to speed the 3 ticks up, or else spread the transactions over an hour or two!
For a much more in-depth guide on marketing view: How to Market?
(See the Resource page if you want to know more about them.)