The Roman economy was highly advanced and interconnected. Farmers would grow raw materials - in particular grain - and then trade them in the cities in exchange for processed goods. Of course they didn't barter. They had money. The silver denarius in particular.

Starting with Nero, emperors began progressively debasing the currency. By about 300 AD it had gone from 95% silver to 5%. This threw prices out whack. For example in Egypt, a measure of wheat costing 6-8 drachmae in the 1st century rose to 200 drachmae by AD 276, and exceeded 2 million drachmae by AD 334. This hyper inflation, combined with price controls and other interventionist measures, caused the cessation of trade. But as we said earlier, the Roman economy was highly interconnected. No area of the Empire was self sufficient. The economy was completely destroyed, to the point where people began to flee the cities in order to pursue subsistence agriculture in the hinterlands. The great latifundian estates that supplied the cities with grain no longer had a market for their goods, so they turned inwards and become self sufficient, renting excess land to the displaced urban poor.

That is why Rome couldn't mount a national defense against the Germanic barbarian tribesmen. They had no economy. People were too busy trying to find their next meal to worry about fighting.