My response will be about Latin Europe. I'm assuming that's what you're inquiring about anyway (given your start date).

It wasn't particularly important TBH. Bartlett's Making of Europe essentially states that as the Roman redistribution system disintegrated, the inhabitants of the frmr Roman Empire were able to compensate for issues of food shortage. Furthermore, the regions of the frmr Roman Empire experienced significant decline in population due to the plagues and strife of the later Empire and certainly on account of the Justinianic plague (although B. Bachrach has asserted that the plague had little effect on Gaul, I remain skeptical). All of this would have alleviated the need for agricultural exchange.

This doesn't mean, however, that such commercial activity was completely absent. Gregory the Great famously saw to the feeding of Rome during his Papacy, but the Italian Peninsula remained (relatively) urbanized throughout the period in question, and thus would have needed more imported agricultural material.

Now, Horden and Purcell would argue that regardless of the population/level of urbanization, the Mediterranean, and therefor the regions of southern Europe, could never have persisted without micro-exchange (very short distance exchange). I have some thoughts about this, but I don't see a particular need to go into them here on reddit.

Honestly, it's after 1000 that we see a distinct need for the exchange of agricultural staples. In “Montpellier et le traffic des grains en Mediterranee avant 1350,” Kathryn Reyerson essentially asserts that during bad years, southern France imported grain, and in good years, it exported this. I think that this was the norm throughout western Europe. Nils Hybel essentially confirm this assumption in, “The grain trade in northern Europe before 1350,” but honestly, more work needs to be done on this topic. Hybel's caveat is that the Low Countries and Denmark had to constantly import grain. Denmark because it didn't have enough arable soil to support its medieval population, and the Low Countries because along with Italy, they were the most urbanized region of medieval Europe.

We know quite a bit about Italy's participation in grain commerce. The cities of Italy outstripped the agricultural production of their hinterlands by the 12th century. In The Two Italies Abulafia essentially argues that the foreign policy of the medieval communes was driven by the desire for grain. By the mid 13th century, many of these cities established specific offices to supply their respective cities with grain.

Hope that answers your question.