Seller risk unfolds over longer horizons and can scale harm across thousands of downstream orders. This chapter models seller-side risk as a lifecycle-from onboarding through listing behavior, fulfillment performance, payouts, and ongoing compliance. It discusses threats such as counterfeit or prohibited goods, manipulation of listings and pricing, chronic fulfillment failures, subsidy exploitation, and networks of related stores that share infrastructure. The chapter emphasizes longitudinal features, cohort baselines, and stability under seasonality and campaign effects, reflecting how seller behavior evolves over time. Interventions are linked to operational levers: onboarding decisions, graduated limits, category restrictions, fulfillment enforcement, and payout controls that reduce loss while supporting legitimate growth. Because seller actions affect marketplace integrity and buyer trust, governance-clear policy definitions, consistent enforcement, and appeals handling-is treated as inseparable from modeling. We also discuss how to measure seller risk programs through leading indicators and how to prevent enforcement from becoming inconsistent or overly punitive as policies evolve. The chapter positions seller models as both risk controls and ecosystem governance tools, protecting buyers while sustaining healthy supply. Taken together, the chapter shows how lifecycle-aware seller models convert detection into durable ecosystem quality at scale.