>To the best of my knowledge, there was not a significant cash crop economy in medieval Europe.

There was a fair bit of specialization and cash-cropping in Rome, though - it's unfair to assume that Rome and the medieval period are the same here, when the key difference is Rome, which 1) acted as basically a giant trade union, economically, and 2) Rome was built around the mediterranean, which obviously made shipping easier than in the middle of Europe.