Silver was used as a medium of exchange during the Viking Age, and there was some minting at Hedeby, perhaps as early as the 820s. But you're right in the observation that Viking Age emporia should probably not be considered coin economies. My best guess is that most transactions were conducted either on direct barter (e.g. this barrel of barley for that stack of pelts) or by referencing a measure of wealth that was well understood. For early medieval societies, this might include not just the value of a certain weight of silver but also cows, slaves, and fields of certain sizes (e.g. I'll trade you iron worth the value of one slave for grain worth the value of half a cow).

Obviously, a society based on barter systems and with few ways to store value (silver stays pretty stable, but cows and slaves get old and die) can hardly be described as capitalism. But barter economies also depend on a notion of personal property that stands at odds with communism. In fact, scholars now generally agree that the terms capitalism and communism (or socialism) aren't very useful for describing societies before the 1800s.

When scholars do make comparisons, they often refer to systems like the Viking Age emporia as pre-capitalist economies—although this sometimes gives the misleading implication that early medieval economics somehow foreshadowed or evolved into modern capitalism. Anything but. Early medieval people had concepts of wealth, property, freedom, labor, and value that were so different from our own that capitalism and communism would probably both seem equally incomprehensible to them.

Conversely, for modern scholars, modern assumptions about economics have caused great difficulties in how we imagine pre-modern societies. For a long time, modern historians simply assumed that premodern societies exchanged goods for their economic value, just as we do in modern capitalist societies. But anthropologists working among Pacific island communities in the early 1900s noted that sometimes people exchanged goods for other reasons like gaining social prestige (Marcel Mauss, followed by Karl Polanyi and Phillip Grierson). This might explain, for example, why some Viking Age elites were so interested in acquiring exotic goods or gifting arm rings to their retainers—all without obvious economic explanations. Viking Age scholars have called this attention to social rather than economic values a 'substantivist' approach, although some are now seeking further alternatives (Dagfinn Skre).

If you're interested, here's two key articles for further reading:

• Phillip Grierson, “Commerce in the Dark Ages: A Critique of the Evidence,” Transactions of the Royal Historical Society 9 (1959): 123-40.

• Dagfinn Skre, "Post-substantivist production and trade. Specialized sites for trade and craft production in Scandinavia c. 600-1000 AD," in James Barrett, ed., Maritime Societies of the Viking and Medieval World (Leeds: Maney Publishing, 2015), pp. 156-170. {online here, although you may need to create a login}

Regarding how notions of capitalism have impacted our ability to study the past, and how historians are seeking to overcome some of the challenges posed by our very modern perspectives on economics, I'd recommend especially:

• Dipesh Chakrabarty, Provincializing Europe: Postcolonial Thought and Historical Difference (Princeton: Princeton University Press, 2000). {examines the pervasive impact of Karl Marx on our abilities to study the past and looks at how we can interrupt this influence}

• Peter Fibiger Bang, The Roman Bazaar: A Comparative Study of Trade and Markets in a Tributary Empire (Cambridge: Cambridge University Press, 2008). {rejects capitalism and communism as the two basic models for economic analysis, looking at the early modern Mughal Empire as an alternative model for understanding how the Roman Empire worked}