<p>With the advancement of the circular economy and information technology, “trade-in” collaborations between e-commerce and recycling platforms have gradually emerged. To boost participation, some e-commerce platforms provide subsidy incentives to users, but poorly designed subsidies may undermine the stability of the partnership. This study focuses on a trade-in collaborative supply chain system involving an e-commerce platform and a recycling platform. Initially, a subsidy model is developed to analyze the subsidy range required for the e-commerce platform to achieve a win–win scenario. Subsequently, a cost-sharing mechanism is incorporated into the existing subsidy model to explore its impact on collaborative performance in depth. Through model analysis, we find that subsidy measures can facilitate a win–win situation between two platforms. The specific subsidy range depends on the recycling platform’s refurbishment-and-resale strategy for waste products and consumers' willingness to retain their products. However, they may also lead to a loss of market share for the e-commerce platform in the trade-in market, and even in the overall market. Furthermore, we determined that introducing a cost-sharing mechanism can indeed optimize collaboration between the two parties, though its implementation path varies under different scenarios. Specifically, in the win–win subsidy scenario, the mechanism does not directly increase profits but achieves a Pareto improvement by increasing system profits and redistributing overflow profits. This requires the e-commerce platform to bear a lower recycling cost. When subsidies are unbalanced, if the subsidy meets certain conditions, the e-commerce platform can restore the win–win situation by bearing a moderate or high proportion of the costs.</p>