If you refer to someone as a shill, you mean that they are paid to sell something or to participate in an activity inorder to persuade others to buy or participate.
[US, informal, disapproval]
He is tarnishing his reputation by being a shill for the tobacco industry.
shill in British English
(ʃɪl)
noun
slang
a confidence trickster's assistant, esp a person who poses as an ordinary customer, gambler, etc, in order to entice others to participate
Word origin
C20: perhaps shortened from shillaber a circus barker, of unknown origin
shill in American English
(ʃɪl)
noun Slang
1.
the confederate of a gambler, pitchman, auctioneer, etc. who pretends to buy, bet, or bid so as to lure onlookers into participating
2.
a person who works energetically to sell or promote something