"Where an agent simply exhibits samples of goods kept for sale by his principal, and takes orders from purchasers for such goods, which goods
are afterwards to be delivered by the principal to the purchasers, and
payment for the goods is to be made by the purchasers to the principal
on such delivery, such agent is generally called a 'drummer' or
— Black's Law Dictionary (HERE)
The most famous commercial traveler in mystery fictiondom is probably Dorothy L. Sayers's Montague Egg, whose unlikely entanglements with crime she featured in not quite a dozen Eggs (see "Resources," below). Because the general conditions were more or less the same for Mr. Egg as they were when this nonfiction article with its amusing anecdotes about traveling salespersons first ran, you could consider it as background reading (and, no, there will not be a test).
======================================================================== "The Commercial Traveller."
By Albert Harris (?-?).
First appearance: The Strand Magazine, June 1907.
Reprinted in The Strand Magazine (U.S.), July 1907.
Short short story (7 pages, with 5 illos).
Online at Archive.org (HERE).
". . . I had hardly got my grip when his feet slipped from off the footboard and he fell back into the darkness. His hat had dropped upon the floor of the carriage. I picked it up, threw it out, and closed the door again."
- Commercial traveler Montague Egg's exploits (HERE) rate highly among detective fiction devotees:
"Sayers wrote eleven Montague Egg stories; they are the first six tales
listed under Hangman's Holiday, and the first five under In the Teeth of the Evidence. These tales are among the best pure detective stories that Sayers wrote. Most of the tales reflect realist school paradigms of detective fiction. Several of the best of these tales center around clocks and alibis. . . . In general, many of Sayers' best plots are found among her shorter fiction.
They tend to have real puzzle plots, in the sense of a initial, well defined mysterious situation that ultimately reaches a clever solution."
— Mike Grost, A Guide to Classic Mystery and Detection (HERE)
HERE), which collects a half dozen Eggs alongside sixteen Lord Peter Wimsey shorts and two nonseries stories:
(2) "Sleuths on the Scent"
(3) "Murder in the Morning"
(4) "One Too Many"
(5) "Murder at Pentecost"
The bottom line: "To serve the public is the aim of every salesman worth the name."
— Montague Egg