UNM press has published some book about the end of the oldstyle trading posts that we used to know--found it--"Navajo Trading: the end of an era," by Willow Roberts Powers; the federal government's finally cracked down. However unfair the system, it was one of the last formalized and regular barter systems in the modern world and probably the last one in this country. I'm not sure what might still be going on in Athabascan and Inuit locales north of the border.