Definition of "barker"

Edmund was barker on sight, like foolish stay-at-home dogs.

1938, Norman Lindsay, Age of Consent, 1st Australian edition, Sydney, N.S.W.: Ure Smith, published 1962, page 42

[…] this fakir was doing a big business, as was shown by the fact that he could afford to hire a barker, who cried continually at the full strength of his lungs: "Come up now, and try your luck! […] "

1893, James Otis, The Adventures of a Country Boy at a Country Fair‎

For every dedicated scientist patiently recording atmospheric pressure and wind speed while shivering at high altitudes, there is a carnival barker with a bevy of pretty girls willing to dangle from a basket or parachute down to earth.

2013 June 7, David Simpson, “Fantasy of navigation”, in The Guardian Weekly‎, volume 188, number 26, page 36

Barney, opening a cupboard, brought forth several articles, which he hastily crammed into the pockets. “Barkers for me, Barney," said Toby Crackit. “Here they are,” replied Barney, producing a pair of pistols.

1838, Boz [pseudonym; Charles Dickens], Oliver Twist; or, The Parish Boy’s Progress. […], volumes (please specify |volume=I, II, or III), London: Richard Bentley, […]

Parkin, the Oxford Street gunmaker, sent me a brace of barkers in silver mountings, with my initials engraved—good for trade, I imagine.

1969, George MacDonald Fraser, Flashman, page 45