“The (best) time to buy an antique is when you see it” is a popular adage in the antiques business. If the prospective buyer waits, the item may be gone and it might not be possible to find it for sale elsewhere.
   
“The time to buy an antique is when you see it” was printed in the Sedalia (MO) Democrat on April 7, 1968. “The time to buy an antique is when you find it” was printed in the Minneapolis (MN) Tribune on August 11, 1969. It’s not known where the saying originated.
 
         
7 April 1968, Sedalia (MO) Democrat, “Antique Show In Progress” by Jennifer Ward, pg. 1, col. 3:
“The time to buy an antique is when you see it,” reads a sign at the fifth annual Sedalia Antique Show which opened Friday at Convention Hall in Liberty Park, sponsored by Mr. and Mrs. Theodore Wells, 1110 West Seventh.
   
11 August 1968, Minneapolis (MN) Tribune, pg. 6H, col. 6 ad:
The time to buy an antique is when you find it.
(Antiques Show.—ed.)
   
1 August 1969, Longview (TX) Daily News, pg. 7-B, col. 5 classified ad:
Remember, the time to buy an antique is when you see it and want it.
(Richardson’s Antique & Oddities.—ed.)
 
4 August 1971, Fremont (NE) Tribune, pg. 22, col. 2 classified ad:
THE TIME to buy an antique is when you find it—and you will find it at THE CELLAR 1343 North Park evenings and weekends.
 
20 January 1972, Wichita (KS) Eagle, pg. 17C, col. 9 classified ad:
The time to buy an antique is when you see it, so don’t take a chance and miss the very item you need.
(Newton International Show & Sale.—ed.)
     
6 July 1972, Kokomo (IN) Tribune,  “Flea Market Has Little Bit Of Everything” by Ron Sentman, pg. 16, col. 4:
A sign above one booth read, “The time to buy an antique is when you see it.”
 
27 May 1973, Cincinnati (OH) Enquirer, “The antiques craze and how it’s growing” by Patricia Mertz, pg. 1-E, col. 1:
In the window of a San Francisco antiques shop, this sign reminds one of the unique qualities of antiques: “The time to buy an antique is when you see it.”
 
30 December 1973, Washington (DC) Star-News, “Astrology as Part of Antiquity” by Orva Heissenbuttel, pg. E-4, col. 8:
Antiques, like horses, become a year older on New Year’s Day. May we all become wiser in 1974, realizing that there are no antique experts, only elderly scholars, and that the time to buy an antique is—when you see it!
   
Google Books
Knock on Wood
By Bruce E. Johnson
New York, NY: Putnam
1984
Pg. 36:
Tip — The best time to buy an antique is when you see it: he who hesitates, loses out.
         
16 December 1984, The State (Columbia, SC), “Savoring antiques” by Starkey Flythe Jr., magazine sec., pg. 11, col. 2:
The signs, those framed buzz words of the business say, “The time to buy an antique is NOW; it might not be there when you come back,” and “If you break it, you bought it.”
 
6 November 1993, St. Petersburg (FL) Times, “Stalking Antiques Series: Days Off” by Ed Merrick, pg. 1D:
Signs everywhere will preach: The best time to buy an antique is when you see it.
 
Remember that, in most instances, the dealer can’t go back into a stockroom and get a duplicate to sell to you. The item you’re considering may be one of its kind at this show or in existence.
 
Google Books
Antique Secrets: How the “Pickers” Find Treasure in Another Man’s Trash
By Joe Willard
Iola, WI: Krause Publications
1998
Pg. 108:
We’ve all seen the sign hanging in an antique store, “The best time to buy an antique is when you see it!” I really believe this is also the case in picking for items. I can’t tell you how many times I have actually left an antique or collectible or junk item at a very good price, deciding to decide later, and upon return find it gone. Yes, there have been times when it’s still there, but it’s been rare.
       
Google Groups: rec.antiques.radio+phono
Transoceanic 1000D
Unrevealed Source
6/7/08
(...)
As they say in the antique business, the best time to buy an antique item that you want is when you find it.
 
Twitter 
Redneck Dad
@redneckdadsays
“The best time to buy an antique is when you see it.”
9:22 PM - 5 Sep 2009
   
Twitter
Jennifer Bonds
@JJ_Bonds
“the time to buy an antique is when you see it” sign in my fav store—will I ever learn this principle??
3:33 PM - 9 Apr 2010
 
Twitter
ae
@SayYesBalto
Replying to @NormalsBooks
My antique dealer parents used to have a little sign up in the shop: “The time to buy an antique is WHEN YOU SEE IT.”
5:44 PM - 28 Dec 2018
 
North Platte (NE) Telegraph
Vintage graniteware has always been high on collectors’ lists
By Judy D. Steele Jun 23, 2019
(...)
As I have always mentioned before (many times), you never know what you might find at the next auction or antique shop. Make sure you visit our local shops as often as you can so we will be able to keep selling locally. Please remember the old saying: “The time to buy an antique is when you see it.” If you wait to go back to get it another day, someone else may have come along and grabbed it up and hauled it away to another town or even another state.