I was looking up some information on Ancient Celts, and one thing that stood out to me was that they conducted trade amongst themselves and with their neighbors. Now, it certainly isn't impossible to trade while being illiterate, but it seems to me to be quite difficult. Not having ledgers or receipts or anything like that would make any long distance trade very cumbersome to say the least. Also, how would you have had taxation?
So the question is, how did they manage to do it? Was it all done by doing calculations in their heads and memorizing all of the different item sales and purchases, or did they use some other system? Did they borrow Greek or Roman writing systems to conduct business with the Greeks and Romans respectively?
All the trade of civilizations that didn't employ neither stable currencies nor writing systems was most likely conducted through barter or by memory.
Many of these populations had an economy much smaller than their most advanced neighbours. We can speculate they didn't have much in the way of a division of labour or differentiated market. There wasn't a clear market tendency and there weren't many occasions of economies of scale, capable of prompting a massive movement or request of goods.
If we assume that most trade occurred within 30 km of the birthplace of the person partecipating in it, in the case of small economies lacking currency and with a shrunk market, he would already know where and to whom to sell his produce, based on his own needs and his acquirer's. He would know by memory how much he had offered and how much he had earned in return.
Celts like their Gaul cousins or the other barbarian tribes eventually acquired Roman or Greek alphabet and customs after being conquered. For what involves their economy before, we can almost only make assumptions.