Mechanism-mediated service markets with polymatroidal feasibility admit efficient, dominant-strategy incentive-compatible (DSIC) allocation, but these guarantees implicitly assume truthful execution by the marketplace operator. Modelling the operator as a strategic player, we establish a credibility trilemma: for single-parameter agents on a non-modular polymatroid, no static sealed-bid mechanism is simultaneously revenue-optimal, DSIC for agents, and credible for the operator. We introduce the Cost of Non-Credibility (CoNC) as a price-of-anarchy-style welfare-loss measure and obtain tight $Θ$-bounds across five topology classes (single-edge, series, parallel, tree, series-parallel), plus a matching upper bound $O(|\mathcal{S}|)$ on general DAGs realised by an $Ω(|\mathcal{S}|)$ witness on the SP-augmented sub-family, turning the trilemma into a structural quantity. Three structurally distinct resolutions follow: public broadcast or deferred-revelation commitment, administrative domain separation under settlement separation and four side conditions, and integrator competition orthogonal to mechanism execution under disjoint actors. An instance-level grounding over the edge-pricing market of Amin et al. confirms the trilemma's robustness on a refereed external setting. The result establishes marketplace neutrality as a first-order design constraint on polymatroidal service markets rather than an implementation detail: where the operator is a strategic player, credibility trades off against revenue optimality and agent incentive compatibility along structurally characterised lines.

翻译：机制中介的服务市场在拟阵可行性条件下可实现高效、占优策略激励相容（DSIC）分配，但这些保证隐含假设市场运营者诚实执行。将运营者建模为策略性参与者后，我们建立了一个可信性三难困境：对于非模拟阵上的单参数智能体，不存在同时满足收益最优、对智能体DSIC且对运营者可信的静态密封报价机制。我们提出非可信性代价（CoNC）作为类似于无政府状态代价的福利损失度量，并在五类拓扑结构（单边、串联、并联、树形、串并联）上获得紧Θ界，同时给出一般有向无环图上的匹配上界O(|S|)，该上界由SP增广子族上的Ω(|S|)下界实现，从而将三难困境转化为结构性量。由此衍生出三种结构性解决方案：公开广播或延迟揭示承诺、结算分离与四类辅助条件下的管理域分离、以及不相关行为体下独立于机制执行的集成商竞争。在Amin等人的边缘定价市场实例层面的验证，确认了三难困境在经评审的外部环境中的鲁棒性。该结果确立了市场中性作为博弈多拟阵服务市场的一阶设计约束而非实现细节：当运营者为策略性参与者时，可信性与收益最优性及智能体激励相容性沿着结构刻画的特征线进行权衡。