Police Commissioner Raymond Kelly says the undercover officer was particularly good at his job - so good that the suspects would fight among each other for his business. He says the weapons sold for about $900 and at least two of the guns recovered were linked to shooting deaths.
Police have arrested 17 people and are looking for three more.
District Attorney Charles Hynes says he won't take plea deals from any of the suspects. He says either they plead guilty to the top charges in the 168-count indictment or they go to trial.
Kelly on Thursday praised the work of the undercover detective, who remained anonymous.