With the rapid development of the platform economy, platforms are evolving beyond their traditional role as intermediaries to actively participate in transactions. Platform-owner entry refers to the practice of platforms directly providing products or services. Grounded in coopetition theory, this study investigates the determinants influencing platform-owner entry in the context of second-hand trading platforms. Based on empirical analysis of secondary data from a leading second-hand trading platform in China, the findings reveal that platforms are more likely to enter markets characterized by higher product prices and greater popularity, aiming to maximize profits. Furthermore, platforms enter to balance supply and demand, thereby addressing customers' product needs. While prior research has predominantly focused on platform-owner entry in first-hand trading platforms, this study bridges a critical gap by examining the unique dynamics of second-hand trading platforms, including the dual roles of complementors and the distinctive attributes of second-hand products. The findings underscore the complementary role of platform-owner entry in the coopetitive relationship between platform owners and complementors. This study provides meaningful insights for future research and practical guidance for second-hand trading platform operators, enabling them to develop effective entry strategies that support sustainable platform growth.