“There's a sucker born every minute”
Although often attributed to P.T. Barnum, some sources claim the phrase is from famous con-man Joseph ("Paper Collar Joe") Bessimer, and others say it was from David Hannum, referring to Barnum's part in the Cardiff Giant hoax. He was exhibiting the "original" giant and had unsuccessfully sued Barnum for exhibiting a copy and claiming it was the original. He 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.
Others claim that late 1860s Chicago "bounty broker, saloon and gambling-house keeper, eminent politician, and dispenser of cheating privileges..." Michael Cassius McDonald, originated the phrase. They say that when McDonald was equipping his Chicago gambling house, his partner was concerned over the large number of roulette and faro games being installed and their ability to get enough players to play them. McDonald then allegedly said, "Don't worry about that, there's a sucker born every minute."
It is the motto of some stock promoters.