Beyond the basic buy/sell flow, I built out four core modules that make it actually usable at scale: request boards (post what you're looking for instead of only browsing what's listed), wishlists, a review system so reputation carries across transactions, and an admin dashboard for oversight. I also automated listing moderation and buyer-request matching with event-driven background jobs, which cut down manual moderation work substantially, important once you're serving 16,000+ potential users and can't manually vet every post.