<p>The increasing penetration of distributed renewable energy highlights the limitations of user-side distributed energy storage (DES), including high costs and low utilization. To overcome these challenges, this paper developed a coordinated operation framework that integrated multiple microgrids (MMGs) with a shared energy storage system (SES). A bi-level energy trading strategy was formulated from the perspective of electricity market participation. At the upper level, the distribution system operator (DSO) acted as the leader in a Stackelberg game to optimize electricity trading prices, aiming to minimize the total system cost. At the lower level, MMGs coordinated with the SES and participated in peer-to-peer (P2P) energy trading, adjusting their scheduling strategies according to market signals. To ensure fairness, a contribution-based allocation method combined with Nash bargaining was introduced, enabling equitable profit distribution among the DSO, MMGs, and SES. Furthermore, a six-mode reserve model for SES was proposed to exploit its full potential for providing ancillary services such as balancing and peak shaving. For efficient computation, the model was solved using a hybrid method that integrated the bisection algorithm with an adaptive Alternating Direction Method of Multipliers (ADMM), which enhanced convergence speed and stability. Case studies verified that the proposed framework effectively reduced system operation costs, improved renewable energy utilization, and achieved fair cost-sharing among participants.</p>