This paper proposes the "platform dominance" green subsidy scheme and develops a sequential game model concluding three players: the buyer, the seller and the C2C second-hand e-commerce platform. We not only solve the optimal green subsidy strategy and the optimal level of integrity construction effort for C2C second-hand e-commerce platforms, but also analyze 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 transaction fee, it should provide green subsidies only to buyers to maximize profits. If not, offering green subsidies to either buyers or sellers will not affect platform profits. 2) The government’s green incentives for C2C second-hand e-commerce platforms should fall within a feasible range, not too low or too high. And charging a proportional transaction fee generates the "leverage" effect of the government's green incentive. 3) When C2C second-hand e-commerce platforms provide green subsidies, imposing proportional transaction fee promotes integrity construction efforts, while charging fixed transaction fee inhibits efforts. If the cost coefficient of integrity construction decreases, the platform should intensify its integrity construction efforts. 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.