The resource system is rate based. There is no stockpile; the player's surpluses will automatically sold in the way to get them the most beneficial resources in return. However, you will have to pay shipping (calculated as a percent cost) so it is a somewhat better (how much depends on the resource) to gather your own. The current resource list is very long adding to the realism and economy, but the trade prevents this from causing an inconvenience. Percent taxes can be set on a per player, and optionally per resource, basis allowing for as much or as little control of the economy as desired. Taxes also allow for embargoes, free trade agreements and things like that.