Many financial transactions are of a fixed-sum nature, meaning that any improvement in the terms of trade for one party comes at the expense of another party. We model how the sales of trading advantages (e.g., data and collocation services) affect traders’ endogenous participation in a market and vice-versa. We show how the magnitude of the externality that a trading advantage imposes on counterparties impacts financial market conditions. In equilibrium, the optimal sales of trading advantages by a monopolist (e.g., data provider or a securities exchange) may result in inefficiently low levels of market participation and trade.