This was not always the case. Economic history is not my specialty, but my understanding is that in the agrarian economy economic transactions typically took place via markets.
The farmer would till his fields and raise his livestock five days a week, and on the sixth day he would load his produce on some form of transportation and take it to the nearest market town, where he would trade it with artisans, fishermen and other farmers who had products that he needed and that his household did not produce.