An old Jewish joke is that Jewish people “don’t buy retail.” I Can Get It for You Wholesale is the title of a 1937 novel by Jerome Weidman and a 1962 Broadway musical that starred Barbra Streisand.
 
Woody Allen, acting in the movie The Front (1976) that was written by Walter Bernstein, said the line:
 
Howard Prince: “In my family the biggest sin was to pay retail.”
 
The usual form of the joke—popular since at least the 1980s— has been:
 
Q: Why did God create Gentiles?
A: So there would be somebody to buy retail.
 
     
IMDb (The Internet Movie Database)
The Front (1976)
Quotes
Howard Prince: Where are you from?
Florence Barrett: Connecticut.
Howard Prince: That’s very ritzy.
Florence Barrett: It’s very proper anyway. I was very well bred - the kind of family where the biggest sin was to raise your voice.
Howard Prince: Oh yeah? In my family the biggest sin was to pay retail.
   
Google Books
The Big Book of Jewish Humor
Edited by William Novak and Moshe Waldoks
New York, NY: Harper & Row
1981
Pg. ?:
Why did God create goyim?
Somebody has to buy retail.
 
Google Books
The Rabbi on Forty-seventh Street:
The Story of Her Father
By Ann Birstein
New York, NY: Dial Press
1982
Pg. 123:
“Daddy,” I said one day, after we had arranged for Hershel’s Goldie’s cloth coat with the fox collar, “do only Gentiles buy retail?”
   
Google Books
How the Platform Professionals Keep ‘em Laughin’
By James Blakely,  Jeanne Robertson and Robert Henry
Houston, TX: Rich Publishing Company
1987
Pg. 240:
Do you know why God created Gentiles? Someone had to buy retail.
 
Google Books
Date with a Plummeting Publisher
By Toni Brill
New York, NY: Worldwide
1995, ©1993
Pg. 165:
Like the old joke says, God made WASPs so there’d be somebody to pay retail; what’s the fun of discount, if there’s nobody out there in the same blouse as you, except that they paid $300 for it in September, and you got it in March for $12.95?
   
New York magazine (1998)
Google Books
Scrapbook: The Wit and Wisdom of Abe Hirschfeld
Excerpts from his current comedy routine.
(...)
. Why did God create Gentiles? Because we Jews needed someone to buy retail.
     
Google Books
Joking Off Again
By Johnny Lyons
New York, NY: Kensington Publishing COrporation
1999
Pg. 88:
Why did God create Gentiles? Without them, who would buy retail?
 
Google Books
Jewish Jocks:
An Unorthodox Hall of Fame
Edited by Franklin Foer and Marc Tracy
New York, NY: Grand Central Publishing
2012
Pg. ?:
I grew up with the belief that God made the Jews as a light unto the nations and made the Gentiles because someone had to buy retail.
 
Haaretz
Opinion: Jews and Capitalism: Call Them Frenemies
A recent rabbinical court ruling in Brooklyn calls attention to the fact that traditional Judaism isn’t so supportive of free markets.
David Rosenberg Mar 30, 2017 4:21 PM
(...)
Only if you think Jews are arch-capitalists who have thrived in the world of business, finance and trade going back to the Middle Ages. “Why did God create gentiles? The old joke goes. “So there’d be somebody to buy retail.”