Bartering allows you to buy and sell resources in exchange for other resources at the market. Resources are traded at exchange rates that vary with each deal. For example, to purchase 100 Food, you may initially have to spend 100 points of another resource type (Wood, Stone, or Metal). By the next deal, however, Food will become more expensive. The rates are global, so if another player buys Food it will be more expensive for you as well. Over time, the exchange rate gradually resets to the default rate.