The man who supplies Doris and Dan Lang daily necessities trades for the protein they grow on their farm, instead of buying it. As the protein the Langs grow is well known for its density, the merchant always gives them an exceptional rate in trade for it. He calls on their farm once a month.
Something Maggie the Almighty talks about getting from the sheriff. when she comes to the Village where Sybille Schmitz sleeps, Apparently Traveling Merchants need permission to do business in a given community on the Frontier.