Brokers can make trades with customers at the same time or after them, with no preference between the two. Market depth and price accuracy are better with trades after customers. If there are more brokers than informed traders, both informed and uninformed traders prefer trades after customers. But if there are fewer brokers, it's better for informed traders to trade at the same time. The model also explains how insider trading affects market volume and liquidity, and how large block sales off the exchange floor can avoid follow-on trading.