“Never give a sucker an even break.”

Source: Reminiscences of a Stock Operator (1923), Chapter IX, p. 103

Source: Lock and Key

Song lyrics, The Dreaming (1982)

To David Irving, from "Hitler's Gladiator: The Life and Wars of Panzer Army Commander Sepp Dietrich" - by Charles Messenger - Biography & Autobiography - 2005 - Page 174

Alaska Young, p. 113
Looking for Alaska (2005)

Source: The Diviners

Commonly attributed to Barnum, there is much testimony of contemporaries that he never actually said this, and in "P. T. Barnum Never Did Say "There's a Sucker Born Every Minute" http://www.historybuff.com/library/refbarnum.html R. J. Brown asserts that it actually originated with a banker named David Hannum, in reference to one of Barnum's hoaxes: a replica of the Cardiff Giant.
Misattributed