I think NPCs should be the main merchants rather than players. For real commerce to take place, you need alot of inventory, which means you have to be stationary. I'm more interested in going to see merchants than asking the other players for what they have in their pockets.
I would like to see a variable economy (like the value of items is different in different places).
Like the scientist pays big bucks for a microscope, but nothing for a chainsaw. A lumber jack would pay alot for the chainsaw, and nothing for a microscope.
That way people don't just vendor dump everything. They can have fun just trading in commodities or whatever. Kinda like "Freelancer" or "Tradewars"