Green’s Dictionary of Slang

(US Und.) the member of a team of cheats, con men or similar group who pretends to be an ‘innocent bystander’ to lure in genuine victims.

1929		V.W. Saul ‘Vocab. of Bums’ in AS IV:5 339: Dooker — One who, while appearing to be one of the audience, is boosting for the house.
1950		Goldin et al. DAUL 63/2: Duker. A handshaker; a come-on man for a confidence swindle mob; a skilled lurer of swindle victims.
2012		W. Keyser ‘Carny Lingo’ in http://goodmagic.com 🌐 Duker — A person handing out "Good for One Free Game" tickets [...] used to get the mark to try a build-up game requiring many plays.