W.C. Field's most famous statement about suckers was "Never give a sucker an even break."
As for the other statement, Wikipedia has this to say:
When Barnum's biographer tried to track down when Barnum had uttered this phrase, all of Barnum's friends and acquaintances told him it was out of character. Barnum's credo was more along the lines of "there's a customer born every minute" — he wanted to find ways to draw new customers in all the time because competition was fierce and people could become bored easily.[citation needed]
Some sources claim the quote is most likely from famous con-man Joseph ("Paper Collar Joe") Bessimer,[1] and other sources say it was actually uttered by David Hannum, spoken in reference to Barnum's part in the Cardiff Giant hoax. Hannum, who was exhibiting the "original" giant and had unsuccessfully sued Barnum for exhibiting a copy and claiming it was the original, was referring to the crowds continuing to pay to see Barnum's exhibit even after both it and the original had been proven to be fakes.
In turn, Barnum's fellow circus owner and arch-rival Adam Forepaugh attributed the quote to Barnum in a newspaper interview in an attempt to discredit him. However, Barnum never denied making the quote. It is said that he thanked Forepaugh for the free publicity he had given him.[citation needed]
Another source credits late 1860s Chicago "bounty broker, saloon and gambling-house keeper, eminent politician, and dispenser of cheating privileges..." Michael Cassius McDonald as the originator of the aphorism. According to the book Gem of the Prairie: Chicago Underworld (1940) by Herbert Asbury, when McDonald was equipping his gambling house known as The Store (at Clark and Monroe Streets in Chicago) his partner Harry Lawrence expressed concern over the large number of roulette wheels and faro tables being installed and their ability to get enough players to play the games. McDonald then allegedly said, "Don't worry about that, there's a sucker born every minute."[citation needed]
The phrase is the title of the opening song in the Broadway musical Barnum, about P.T. Barnum's life.
[edit] History
The earliest appearance of the phrase in print is in the 1885 biography of confidence man Hungry Joe, The Life of Hungry Joe, King of the Bunco Men.[2][3] Another early appearance is in Opie Read's 1898 novel A Yankee from the West.[4]
According to David W. Maurer, writing in The Big Con (1940),[5] there was a similar saying amongst con men: "There's a mark born every minute, and one to trim 'em and one to knock 'em". Here 'trim' means to rip off, and 'knock' means to persuade away from a scam. The meaning is that there is no shortage of new victims, nor of con men, nor of honest men.
In the John Dos Passos novel The 42nd Parallel, the quotation is attributed to Mark Twain.