They used coinage, or tally sticks, or any kind of common unit of exchange they agreed on. The important thing was having a medium of currency to enable trade; what was fairly uniform was the realisation that the form itself was not that important. It didn't matter if the medium was valuable in and of itself. By the early medieval age coinage was well established.

That's a large complex topic. Barter was always in use, but as a substitute of money as we understand them, couple of different things were used in different location across the globe. Generally speaking these were objects/materials that had it's own high value, were durable, easy to store and transport. Metal in different forms, usually ingots of different shapes. That includes not just precious metals but also iron, cooper and bronze. That eventually led to actual coins. Cloth was used, again, in different forms, including actual costumes and garments. Today cloth is cheap, but in pre-industrial era cloth was very laborious to make and expensive. Animal skins were used, especially luxury ones. More exotic objects could be used, usually in some smaller isolated and more primitive societies: sea shells, coral beans -usually in placed where these were been imported, not where they were abundant.

I don't normally recommend David Graeber's book, Debt. But this is one topic that it was actually really good on. It talks about the way people traded when money was scarce, which is most of human history in most places.

There's a lot of valid criticism of the book so I would maybe check out the wikipedia entry on it to get the outlines of the debate and to help read it critically b/c Graeber is a great writer and he's can be very convincing if you're only getting his side of the story.

https://en.wikipedia.org/wiki/Debt:_The_First_5000_Years