Would a distributed marketplace even charge a fee? I would assume all users would be happily compensated by the existence of the marketplace. The lack of fees also increase margins/decreases price

Somebody has to pay to host the thing, run it and so forth.

Quite right. Still, I think this whole libertarian concept is probably based on the idea of _someone_ making some money somewhere along the lines. :)

I assume the mediators would need to be paid, at the bare minimum, probably moderators to filter out junk and spam and whatever other unforeseen circumstances pop up.