In many electricity markets, declining feed-in tariffs have made grid export increasingly unattractive for residential solar prosumers, while retail electricity prices remain high. Peer-to-peer (P2P) energy trading offers a direct alternative, but it requires a dedicated infrastructure layer for real-time bilateral matching, automated settlement, and tamper-proof transaction records, for which blockchain is widely proposed. Deploying such infrastructure must be economically justified by the community's actual trading potential. A critical and underexplored question is whether trading potential survives as communities become prosumer-heavy, since under fixed role assignment all households eventually end up on the supply side with no buyers remaining. This paper addresses these gaps by proposing the Energy Trading Potential Index (ETPI), a normalized data-driven metric that quantifies the structural impact of flexible role switching on community-level trading potential, where prosumers dynamically join the buyer side whenever they are in energy deficit. The P2P market is modeled as a generalized bipartite graph and pairwise interaction scores aggregated over trading rounds compute the ETPI in [0,1]. Simulation results using the PRECON residential dataset and NREL PVWatts solar profiles show that for the (1:9) prosumer-heavy mix, the flexible policy achieves an ETPI of 0.61 versus only 0.15 under the static policy, a fourfold improvement that the static model entirely misses. The ETPI framework serves as a lifecycle decision-support tool for evaluating and monitoring P2P energy trading infrastructure.

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