I second Russ' suggestion of Common Ground on the Hill, having just been there this past weekend. Very nice festival, quite a few vendors. They sell performers' CDs at the Festival's own tent, but I can't imagine they'd object to another music vendor at this festival which is heavily geared toward traditional folk. They also run "Traditions Weeks" on either side of the Festival, consisting of classes in all kinds of folk arts. No idea what they do -- or don't do -- as far as vendors go for those weeks. But do check out the Festival.