WordNet records no synonym and no antonym for shill. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a decoy who acts as an enthusiastic customer in order to stimulate the participation of others
act as a shill
“This man shilled and bid for an expensive carpet during the auction in order to drive the price up”
Synonyms, antonyms and the broader and narrower relations for shill come from Princeton University’s WordNet. Full sources and licences.