This paper proposes the “platform dominance” green subsidy and develops a sequential game model involving three players: the buyer, the seller, and the C2C second-hand e-commerce platform. It not only solves the optimal green subsidy strategy and the optimal level of integrity construction effort for C2C second-hand e-commerce platforms, but also analyzes how various factors, such as charging rules, influence the optimal decisions and profits. The results show that: 1) If the C2C second-hand e-commerce platform imposes a proportional fee, it should provide green subsidies to buyers. If not, offering green subsidies to either buyers or sellers would have the same effect on platform profits； 2) The government’〖KG-*5〗s green incentives for C2C second-hand e-commerce platforms should fall within a feasible range, not too low or too high. Additionally, charging a proportional fee could generate the “leverage effect” of the government’〖KG-*5〗s green incentive； 3) When C2C second-hand e-commerce platforms provide green subsidies, imposing proportional fee could promote integrity construction, while charging fixed fee could inhibit integrity construction. If the cost coefficient of integrity construction decreases, the platform should intensify its integrity construction. These findings contribute to advancing the internalization of green consumption externality by platforms in the context of the “Carbon neutrality”. Additionally, they provide valuable practical insights for the integrity construction of information intermediary platforms.