shill n.

"one who acts as a decoy for a gambler, auctioneer, etc.," by 1911, in newspaper exposés of fake auctions, perhaps originally a word from U.S. circus or carnival argot and a shortened form of shilaber, shillaber (1908) "one who attempts to lure or customers," itself of unknown origin and also a surname. Carny slang often is deeply obscure. The verb, "act as a shill," is attested by 1914. Related: Shilled; shilling.

The auction game, as practice in Chicago on South State street, for instance, is a sordid affair, run according to cut and dried rules which admit of no freshness or originality. The list of employees is made up of one backer, or proprietor, two auctioneers, one pretty girl cashier, and from two to ten "shills," whose business it is to stand around in the crowd and make fake bids for the articles on sale. [The Chicago Sunday Tribune, May 12, 1912]