There's a bit of adjustment required in your concept of those early times. First, if you are thinking , say, 1300 AD in Europe, there might not have been much to buy: much of what I see in this room I am in right now would have existed, other than a table and a chair. Even the books would be rather expensive, might have actually been chained in place so they didn't get stolen by readers.

It was not unusual to bury coins to hide them- we know this, because sometimes they come to light today. But still, money was used sometimes in large quantities by merchants, kings, bishops. Iron-bound lockable wooden chests survive from the period for holding coins..at the Palace of the Pope in Avignon, you can see one set of rooms that had spaces beneath their paving stones for secure storage of money, and presumably there would have been guards keeping watch on top of those paving stones.

The Pope could travel with those guards. Of course it was really hard to travel long distances through foreign lands without guards, if you want to carry lots of money, and this was an essential part of merchant houses, like the Medici and the Fuggers, and also of the order of the Templars: if you were, say, going on a pilgrimage from London to Jerusalem, you could deposit money with the Templars in London and carry letter of credit for the Templars in Jerusalem. This made the Templars quite wealthy, as they would take a fee.

If you jump ahead to the US circa 1790, you will find something similar, but expanded. People would often write notes in lieu of money: Jones bought a load of firewood from you, he gives you a note saying he will pay two dollars. You buy some bricks from Smith, you owe him three dollars. You show him the note from Jones, and he says he'll credit you a dollar and fifty cents for it. And of course, if somebody starts a bank, the bank issues its own notes. The the government says it will issue notes...and you see where this is going.