MarketSpan: A Hallucination-Free Span Extractor and the Measured Backbone of an Auditable Mechanical-Keyboard Price Index (MMI)

Mechanical Market Intelligence (MMI) transforms 14 years of mechanical-keyboard classifieds (715k posts, 249k extracted listings) into a price-intelligence platform. It answers practical marketplace questions: current and historical value, trade value, future price, fair-value assessment, demand trends, item correlations, and trader relationships. At its core is MarketSpan, a 142M-parameter extraction model distilled from Llama-3.3-70B that is hallucination-free by design—it only copies spans from source posts and never generates information. Trained on a free Kaggle T4 in under 3 hours, it achieves 0.912 price-span F1, 0.966 post-type accuracy, and 0.746 price-item binding accuracy, significantly outperforming a from-scratch baseline. Built on this auditable foundation, MMI provides hedonic price indices, barter-graph valuations, cost prediction and forecasting, anomaly/scam detection, demand analytics, and transaction-network reconstruction. Every valuation, forecast, and deal flag is fully traceable to verbatim text in real marketplace listings, making the entire system transparent and verifiable.