Individuals and sole proprietors in developing countries like Thailand have been reaping the financial benefits of social media technologies that have created new trading channels. Commercial activities conducted by users of social networking sites among users have grown significantly. A proliferation of new vendors has forced existing ones to improve their business for competitive advantage. Systematic and effective implementation and improvement have taken place thanks to tools like the maturity model that allow for business capability self-assessment. Although several studies describe the maturity of social media usage in organizations or businesses, few of them focus on the C2C business model. This study presents a maturity model of C2C s-commerce for assessing stage of growth of C2C s-commerce. The methodology included both secondary and case study research. The model consists of five maturity levels (ad hoc, active repeatable, proactive functional, pre-emptive managed, and optimized). Their characteristics have been researched through four dimensions (business environment, people, process, and technology). The proposed maturity model helps online vendors conduct business activities by reducing business risks while supporting consumers by the minimizing risks associated with purchasing.