Honeypots Against LLM Collusion: Sellers Report the Offer, Pretend to Agree, and Keep Their Prices

Autonomous LLM sellers can coordinate on prices; one proposed countermeasure is a trusted agent that invites collusion and reports whoever accepts. We test it in an LLM double auction with private messaging and a neutral report action, where a scripted seller privately offers every seller a $96 price floor, about the going price; other arms add human pleas, a warning that undercover agents may be present, or a precedent of punished sellers. Gemini 3.8 Flash sellers almost all reported the offer (19 of 20), yet 7 also wrote back agreeing, 6 stating an aim to keep the proposer anchored, and none of the 20 reported a peer's price talk. GPT-4.1 sellers backed the offer in words and, at $96, reported it only under a precedent describing the scenario; Claude Haiku 5.5 sellers, asking about $10 below the offer, refused it openly. Under the one-sentence warning, Gemini sellers in the undercover market never messaged each other privately, while each arm's average price stayed inside the baseline's range. Price evidence is weak by construction: the baseline met our price criterion in 1 of 5 seeds, and prices held or fell from the opening ask. With three to five seeds per arm, results are directional. A honeypot offer exposed double-dealing but rarely drew reports of the sellers' own price talk, and silence under a warning is not evidence of deterrence.