From Jack Weatherford, Genghis Khan and the Making of the Modern World, p. 221:

The expansion and maintenance of the trading routes did not derive from an ideological commitment of the Mongols to commerce and communication in general. Rather, it stemmed from the deeply rooted system of shares, or khubi, in the Mongol tribal organization that had been formalized by Genghis Khan. Just as each orphan and widow, as well as each soldier, was entitled to an appropriate measure of all the goods seized in war, each member of the Golden Family was entitled to a share of the wealth of each part of the empire. Instead of the salary paid to non-Mongol administrators, the higher-ranking Mongol officials received shares in goods, a large part of which they sold or traded on the market to get money or other commodities.

I picture the Roman empire working the same way. Goods are seized or expropriated, then traded. Same with the Spanish empire.

I picture the early British empire as a hybrid or transitional system between a plunder economy and a trading economy. Initially, quasi-private entities (Francis Drake, the East India Company) were given licenses to plunder, as long as they were acting in the interest of the crown. Eventually, this evolved away from plunder and toward modern trade.