"The customer is always right" has also long been attributed to John Wanamaker (1838-1922). Wanamaker's stood on Astor place, where the K-Mart is today. If Wanamaker didn't coin the proverb, he at least used it.
15 June 1912, Indianapolis (IN) Star, pg. 8, col. 2:
One secret of Marshall Field's success was the motto he enforced, "Remember that the customer is always right."
23 August 1912, Lincoln (NE) Daily News, pg. 4, col. 2:
A certain successful clothing house deliberately tells its clerks to remember that "the customer is always right."
11 October 1912, Trenton (NJ) Evening Times, pg. 4, col. 2:
Our motto is: "The customer is always right." - and we live up to our motto.
(The P. & O. Shop - ed.)
23 June 1914, Christian Science Monitor, pg. 2 (from A. A. Shryer of Detroit):
"If 50 per cent of our business men could be made to really understand how infinitely profitable it is to practice the axiom 'the customer is always right,' profits from advertising would increase far beyond any point of concern regarding waste."
2 July 1915, New York Times, pg. 3 ad:
Riker-Hegeman Drug Stores. Drug Stores with character and a
conscience. The customer is always right.
43 New York Stores.
14 July 1918, Chicago Daily Tribune, pg. 23:
However, the motto of the store (James & Younger - ed.) was, "The customer is always right," so, all right.
17 November 1918, Chicago Daily Tribune, pg. E10:
The business (Montgomery Ward - ed.) began in one room twenty feet square at 246 Kinzie street, Chicago. From there the firm announced by mail that they were able to ship goods diret to the customer, and that the customer should be the judge of his own satisfaction with these goods and the prices paid for them. Thus they became the pioneer mail order merchants of America, with a business based on the theory that "The customer is always right."
26 July 1954, Chicago Daily Tribune, pg. 22:
"Give the lady what she wants," said Marshall Field, and thus began the tradition that the customer is always right.

