Mark Koyama has two more follow-ups, here and here. In the first, he writes
Roman transport links were as good as any that existed in Europe until the late 18th or even the 19th centuries.
This may address a point that I was going to make, which is that you don't see trade in basic foodstuffs on a large scale until nearly 1850. For much of history, we had trade in spices, precious metals, sugar, and some textiles. But my sense is that basic food rarely traveled more than a few miles until the late 19th century.
But now we go back to Rome and we allegedly see grain being shipped over long distances in large quantities. At the very least, this requires efficient transportation.
So let us grant that transportation during the Roman empire was remarkably good compared to what followed for many hundreds of years. The question still exists whether food came to Rome through market exchange or by imperial taxation. My bet would be on the latter, and Koyama seems prepared to agree.
Taxation could give rise to all sorts of complex financial transactions, including promissory notes backed by future tax collections. You could build up a rich set of markets where the soldiers and tax collectors in Rome trade with one another using coins as a medium of exchange. But the foundation of these markets is plunder.
After Tamerlane makes it appear that the ratio of plunder to production remained high well into the 18th century. Only when Great Britain established hegemony at sea did the movement of merchandise evolve from a state-run plunder-monopoly to a competitive enterprise.
The Marxist view of globalization is that it never ceased being all about plunder. Adam Smith had a different view, but in his day the world may actually have been more Marxist than Smithian.