<p>This paper considers the quality effort behavior in the secondhand market, which includes a secondhand goods trading platform and a secondhand product supplier. The platform and the supplier sell products with improved quality to consumers through different sales modes, i.e., the agency selling mode and the reselling mode. We built a game model of the supply chain and studied the optimal pricing and quality effort decisions as well as the preferences of the platform and the supplier under different sales modes. For the platform, both quality effort costs and product quality fluctuation affect its preference for the sales mode, while the supplier's choice is influenced by price sensitivity, platform reputation, and quality effort costs. A win–win situation is achieved only when the platform’s reputation is high and its quality effort costs are low. We find that when the platform's reputation is high and its quality effort costs are low, the reselling mode enhances consumer experience and welfare, aligning with both platform and supplier preferences. Additionally, the platform benefits from cooperative quality efforts, but these do not always profit the supplier, especially in the reselling mode when quality fluctuation is high. Cooperative efforts increase platform profitability and transaction volume but do not always improve social welfare. Finally, we extend the model to include competition between new and secondhand products, showing that secondhand goods can capture market share from new products by pricing or expand the market through quality effort.</p>