A system and method are provided for managing multiple investor accounts by rapidly computing and displaying the potential tax and gain/loss implications of proposed buy and sell transactions involving a single security. The invention allows users to identify criteria including tax selection liability rules and gain/loss rules for the proposed trades, and generate a trading ("What-If") report identifying the tax lots in each account that would satisfy the selection criteria. A failure report identifying tax lots that do not satisfy the selection criteria allows the user to either transfer such tax lots to the trading report or defer trading using messages transmitted in the future as a reminder to review the proposed transaction at a more appropriate time.