A computer implemented method using aggregation for enabling a user to create and trade a plurality of market tradable assets or liabilities as a single, customizable investment portfolio. An embodiment of the invention includes determining, based on an order to trade a portfolio from a user, a plurality of distinct market tradable assets or liabilities to be transacted in a market for each of the distinct assets or liabilities in a plurality of transactions for the user, aggregating the plurality of transactions for the user with a plurality of transactions for one or more other users over an applicable characteristic of the plurality of assets or liabilities, wherein the aggregating includes aggregating single shares, odd lots and/or fractional shares using a computer, and executing one or more trades based on the aggregating to implement the order to trade the portfolio.