It’s easy to forget commerce in early medieval times. Yet that sign of civilization existed despite wars, disease, and famine.
In an age when only a few could write, most sales went unrecorded, but those that were used monetary units to say what the trade was worth. A parcel of land was described as being worth 6 deniers or that value in goods such as food and animals.
Despite a lack of formal schooling, early medieval people knew how to bargain, although they often bartered for what they wanted. See my post at English Historical Fiction Authors for a sampling what goods were prized in the Dark Ages. (Hint: Fur was a status symbols in those days, too.)
Daily Life in the World of Charlemagne by Pierre Riche