We propose a Marshallian model for price and allocation adjustments in parallel continuous double auctions. Agents quote prices that they expect will maximize local utility improvements. The process generates optimal allocations in the limit. In experiments designed to induce CAPM equilibrium, price and allocation dynamics are in line with the model’s predictions. We identify, theoretically and empirically, a portfolio that is closer to mean-variance optimal throughout equilibration. This portfolio serves as a benchmark for asset returns even if markets are not in equilibrium, unlike the market portfolio, which only works at equilibrium. The theory has implications for momentum and liquidity.