“Listen up. I am not a shill!!”
The Oxford English Dictionary does list the word in Tomasky’s sense of ‘a decoy or accomplice, esp. one posing as an enthusiastic or successful customer to encourage other buyers, gamblers, etc,’ which it dates to 1916. It adds, from 1976, ‘One who poses as a disinterested advocate of another but is actually of the latter’s party; a mouthpiece, a stooge’. The usage is described as ‘slang (chiefly N. Amer.), [Origin unknown.]’
Interestingly, Tomasky later uses the verb shilling – what kind of a coinage is that?