"The Man Who Sold Rope to the Gnoles" by Margaret St. Clair

First appeared in Fantasy and Science Fiction.  Reprinted (a few of which are genre retrospectives) by Judith Merril, Alfred Hitchcock, Helen Hoke, Martin H. Greenberg, Terry Carr, Isaac Asimov, David G. Hartwell, Kathryn Cramer, Robert Silverberg, Jeff VanderMeer, Ann VanderMeer, Betty M. Owen.

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.

*Spoiler*

 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.

Like "Brightness Falls from the Air", the story deals with St. Clair's preoccupation with global connectivity and consciousness.  In this case, pay attention to the signals those of another culture are giving off.