Mortensen is an excellent salesman. He goes out to the gnoles--an unpleasant, rarely visited people who live among the trees on the edge of Terra Cognita--and sells them rope, which no one has before. They may not even need rope. But he makes a faux pas.
The key to this tale appears early on when he reviews his book on salesmanship:
"[I]t may be that his failure to put 'tact and keen power of observation' on a even footing with the other attributes of a salesman was responsible for what happened to him."
Being a man of conscience, he doesn't accept the jewel given as payment. Instead he grabs something that appears of lesser value off a shelf. But they are the eyes of the gnole, so they eat him.