Cartel Talk, Competitive Prices: Opening Anchors—Not Communication or Oversight—Drive Apparent LLM Seller Collusion in Double Auctions

LLM sellers in continuous double auctions are reported to coordinate high prices, especially when they can talk. Project 3.4 asks what such a coalition does when it learns it is being watched. We first tested whether the coalition exists. Across 151 market sessions with GPT-4.1, Qwen3 and Claude sellers and LLM buyers, we find: (1) the starter environment’s rule-based buyer is frozen at $80, invalidating cheap runs; (2) final prices are set by the hard-coded opening quotes. GPT-4.1 sellers hold their opening ask (r = 0.995, n = 48), Qwen3-235B markets settle at the buyers’ opening bid, and a Qwen market “passes” the built-in collusion test when buyers merely open high. (3) Despite explicit cartel language from both families, seller communication barely moved prices: paired-seed effects ranged from -0.23 to +1.08 dollars across 10 settings, under 11% of the $10 competitive–cartel gap. (4) When sellers are told they are monitored, overseer coordination scores fall from 3.87 to 2.0–2.5 and messages shorten, but neither prices nor ask dependence rise, so there is no channel movement by the track’s standard. (5) Under excess supply, prices stay $8–11 above the competitive $80, but an excess-demand control also fails to rise, so within 15 rounds the market barely responds to supply and demand and cannot reveal tacit collusion. A stronger seller (Claude Sonnet 5.5, one seed) does track the spare seller and cuts prices about five times faster than GPT-4.1. We recommend paired-seed causal metrics, neutral openings and market-responsiveness controls.