Yes, the physical evidence does suggest that traditional British balladry derived from commercial broadsides, but that reflects a problem inherent in historical sources. "It's not in the documents, it didn't happen" is the way someone once put it. But there's plenty that happened and wasn't written down, for myriad reasons. There is evidence, cited by Jim and others, that suggests a creative role for 'the people' (those of whom who were not broadside hacks themselves, that is) but there will always be more evidence to privilege the role of broadsides simply because we have piles and piles of them in the Bodleian Library (and elsewhere) and we don't have tape recorded evidence from the 17th century from singers giving their side of the story. The weight of evidence, as I understand it, suggests a reciprocal role between print and oral transmission, between individual and group creativity on one hand, and commerce on the other. You can see a similar process with American Hillbilly music of the early 20th century, with commercial recordings reflecting both folk traditions and mass culture, and interaction and cross-fertilization between the two.