I think it comes down to respect and social status. In the times that the marketplace was a place of meaningful interaction, the vendors were merchants or tradesmen selling their products and those were relatively respected stations in life. They weren't nobility, but they weren't at the lowest strata either.
Today, retail is a near-zero-skill job that's manned by people who are either young or uneducated. When I worked retail between high school and college, I remember there being a glaring mix of people like me who were transitioning through it and lifers who'd been there for a decade or more. And you could see it in the interactions with customers. When dealing with the younger employees, customers often adopted an avuncular style that's used when dealing with people of the same class but different generation. But when I'd see people talk to the lifers, the interactions were more mechanical and less personal. It was almost as if customers felt that there wasn't any shared context and so a conversation that went beyond the immediate transaction was pointless.
I contrast this with my experiences going to farmer's markets. There, vendors are viewed much more as equals. They're largely respected and often own their own business. It's much more common to see people having conversations that feel like they're between equals than it is to see those conversations at a traditional supermarket.