Entry in progress—B.P.
 
 
Wikipedia: Jay GouldJason “Jay” Gould (May 27, 1836 – December 2, 1892) was an American financier who became a leading American railroad developer and speculator. Long vilified as a stereotypical “robber baron”, modern historians have discounted various myths about him and more positively evaluated his career. 
 
1 July 1892, Wall Street Daily News, pg. 1:
PROFESSIONALS agree"that it is never safe to sell a dull market;” hence they are contenting themselves with trying to talk the market down, rather than selling it down. 
 
25 July 1892, Wall Street Daily News, pg. 4:
Shrewd bears rarely sell a dull market short. 
 
19 May 1896, Wall Street Daily News, pg. 1:
The greatest bear point on speculation is its stagnation, but Wall street knows well, and years of active use have proven its usefulness, that “it is never safe to sell a dull market.”   
 
29 December 1897, Galveston (TX) Daily News, pg. 6, col. 6:
Jay Gould said: “Never sell on a dull market when conditions are good.” 
 
12 October 1898, Wall Street Daily News, pg. 2:
The professional room traders have evidently lost sight of the old familiar saw in Wall street, “never sell a dull market,” as they are at present engaged in just such an operation. 
 
22 January 1902, Decatur (IL) Review, pg. 7, col. 3:
“Never sell in a dull market,” said the late Jay Gould. 
 
31 March 1905, New York (NY) Times, pg. 14:
Two old adages were frequently quoted in brokerage offices yesterday. One was that it would not do to sell the market when the sap was running up the trees, and the other was that it was dangerous to sell a dull market. 
 
3 May 1905, New York (NY) Times, pg. 13:
There is an old adage that it is dangerous to sell a dull market, but when that market combines dullness with stubborn resistance to selling orders then it becomes doubly dangerous.   
 
10 June 1905, New York (NY) Times, pg. 11:
Many persons in Wall Street yesterday had recourse in the old tradition that it was a bad plan to sell a dull market short. 
 
10 July 1907, Wall Street Daily News, pg. 1:
Jay Gould’s famous aphorism is recalled. “Never sell a dull market.”   
 
Google Books
The Stock Market Barometer:
A Study of Its Forecast Value
Based on Charles H. Dow’s Theory of the Price Movement.
With an Analysis of the Market and Its History Since 1897
By William Peter Hamilton
New York, NY: Harper & Brothers
1922
Pg. 146:
In the old days Wall Street formulated a number of maxims for itself, and one of these was, “Never sell a dull market.” It is bad advice in a major bear swing, for the markets then will become dull after a sharp rally, and experienced traders will accordingly put out their shorts again.     
 
19 April 1929, New York (NY) Times, pg. 38:
Old Axiom Proves True.One of the old-time wisecrackers—it has been variously attributed to the Hon. James Fisk Jr., to Russell Sage and to possibly a half score others—once laid it down to a safe trading rule “never to sell a dull market.”
 
14 January 1930, New York (NY) Times, pg. 40:
On the other hand, those who foresee a better market in the future are repeating the age-old Wall Street maxim that it is never wise to sell a dull market. 
 
Google Books
Introduction to Wall Street:
A Practical Guide Book for the Investor Or Speculator
By John Francis Fowler
New York, NY: Harper & Brothers
1930
Pg. 175:   
It is apt to be least active at or near the bottom, giving rise to an old Wall Street maxim: “Never sell a dull market short.” 
 
29 June 1930, New York (NY) Times, pg. 35:
One of Wall Street’s oldest axioms, “Never sell a dull market,” has thus been shattered, and old traders say they do not know what to expect.