“If you don’t know coins, know your dealer” (coin collecting adage)

“If you don’t know jewelry, know your jeweler” is a saying that dates to at least 1927. “If you don’t know (your) coins, know your dealer” is a similar adage that has been applied to coin collecting.
 
“If you followed the slogan another industry uses — ‘If you don’t know your coins then know your dealer’ — you probably are not concerned” was cited in The Numismatic Scrapbook Magazine in 1965. “There’s an old industry saying: If you don’t know your coins, know your dealer” was cited in Forbes magazine in 1974.
 
     
Google Books
16 April 1949, The Billboard, pg. 24, col. 3:
Offers choice of eight nationally advertised brands, advises “know your dealer if you don’t know television.”
     
Google Books 
The Numismatic Scrapbook Magazine
Volume 31
1965
Pg. 459:
If you followed the slogan another industry uses — “If you don’t know your coins then know your dealer” — you probably are not concerned.
   
Old Fulton NY Post Cards
17 January 1973, Oswego Valley News (Fulton, NY), “Coins” by George McLaughlin, pg. 3, col. 3:
It all goes back to the old adage, if you don’t know your coins, know your dealer.
 
Google Books
Forbes
Volume 114, Issues 7-12
1974
Pg. 29:
There’s an old industry saying: If you don’t know your coins, know your dealer. It’s a good rule— except, how can you know your dealer? There is no training, testing or licensing required.
 
Google Books
Unofficial Misstruck and Counterfeit U. S. Coins
By D. Taxay
New York, NY: Macmillan General Reference
1975
Pg. 58:
To put it succinctly: be cautious, view each coin with the sum total of your knowledge and — if you don’t know your coins, then know your coin dealer!
 
26 January 1981, Playground Daily News (Fort Walton Beach, FL), pg. 3, col. 2 ad:
If You Don’t Know Coins, Know Your Coin Dealer.
COIN INVESTMENTS
 
9 February 1984, Los Angeles (CA) Times, “Your Coins: Glendale Numismatic Show a Departure From the Past” by Don Alpert, pt. 5, pg. 26:
As (coin dealer Joel—ed.) Rettew cautioned: “There’s an expression in the jewelry business. If you don’t know jewelry, know the jeweler. The same is true in numismatics. If you don’t know coins and metals, know the dealer.”
 
30 October 1996, Iola (KS) Register, pg. 10, col. 1 classified ad:
“If you don’t know your coins, know your coin dealer”
 
YouTube
If you don’t know coins, you better know your dealer.
McG silver
Published on Jun 14, 2016
An interesting article about the rise of fake coins.
   
NBC News
JUN 13 2016, 12:09 PM ET
Glitters, but Not Gold: Fake Gold and Silver Coins ‘Flooding’ Market
by HERB WEISBAUM
(...)
“There’s an old saying that can help buyers avoid problems: If you don’t know coins, you better know your dealer,” said Dana Samuelson of the Professional Numismatists Guild.