James Dines, editor of The Dines Letter (online investment advice), said in 1983: “If you hear that everybody is buying a certain stock, ask who is selling.” The investment saying has appeared in many quotations collections.


Investment Trivia/ Stock Market Trivia/ Wall Street Trivia
More Famous Investment Quotations
“If you hear that everybody is buying a certain stock, ask who is selling.” James Dines, Investment newsletter writer (1935- )

15 April 1985, Orlando (FL) Sentinel, “Understand the risks—before you invest” by Dick Marlowe, special section, pg. 6:
And the more modern-day financial wizard James Dines said, “If you hear that everybody is buying a certain stock, ask who is selling.”

Google Books
The New Official Rules:
Maxims for muddling through to the twenty-first century
By Paul Dickson
Reading, MA: Addison-Wesley Publishing Company
1989
Pg. 62:
Dines’s Reminder. If you hear that everybody is buying a certain stock, ask who is selling.
-- James Dines, quoted in the Baltimore Sun, November 9, 1983