Classical optimal auction theory assumes that bids reach the seller directly. We study how this picture changes when a revenue-maximizing intermediary controls access to the seller's auction. Motivated by blockchain auctions, online platforms, and other intermediated markets, we consider a single-item auction with independent private values and a monopolist intermediary who can decide which bidder messages are forwarded to the seller. We establish approximation guarantees and impossibility results across three timing models: seller-first, intermediary-first, and simultaneous. In the seller-first model, arbitrary deterministic seller mechanisms collapse to posted-price mechanisms, and the intermediary's best response is a shifted Myerson auction. This yields a sharp separation: for regular distributions, the seller's revenue can be arbitrarily small relative to the no-intermediary optimum, while for $α$-strongly regular distributions, posted prices recover a constant fraction of the optimum with a tight dependence on $α$. We further show that timing matters: neither Stackelberg order uniformly dominates, and simultaneous play can leave both parties unboundedly worse off than in either sequential model.

翻译：经典最优拍卖理论假设竞价直接到达卖方。本文研究当存在追求收益最大化的中介控制卖方拍卖的访问权限时，这一图景如何改变。受区块链拍卖、在线平台及其他中介市场的启发，我们考虑具有独立私人价值的单一物品拍卖，其中存在一个垄断中介，该中介可决定哪些竞标者信息被转发给卖方。我们在三种时序模型（卖方优先、中介优先和同步）下建立了近似保证与不可能性结果。在卖方优先模型中，任意确定性卖方机制退化为标价机制，而中介的最优反应是偏移的Myerson拍卖。这产生了显著的分离效应：对于正则分布，卖方收益相对于无中介最优值可任意小；而对于$α$-强正则分布，标价可恢复最优值的常数份额，且该份额与$α$存在紧系。我们进一步证明时序选择至关重要：任一Stackelberg顺序均非绝对占优，且同步博弈可能导致双方境况相较于任一序贯模型无界恶化。