Back when the liver eater brought his plews in were there more than 1 buyer?

There were multiple traders at most of the big rendezvous in the Green River area. Each fur company typically sent its own train of goods. And there were sometimes also free traders.

Company trappers were required to "sell" their furs to their employer. Free trappers could trade with whoever they wanted.

The golden days of the fur trade were gone by the time John (Jeremiah) Johnson got to the mountains in the late 1840s. There hadn't been a real rendezvous in years. 1825 to 1840 for rendezvous. He likely sold his furs at local trading posts. In later years there is record of him making a portion of his income as a woodhawk, cutting firewood for the Missouri River steam boats. . He also did some scouting for the Army. During the Civil War he fought for the Union in the Western Missouri Campaigns. He also, along with a character named X. Beidler reportedly made whiskey and sold it to Indians. Spent his later years around Red Lodge, Montana and died in a veterans home in Los Angles in 1900 at the age of 75.