In one embodiment, the present invention is a method and apparatus for matching buyers and sellers in an anonymous online marketplace. One embodiment of a method for defining a community of network users includes collecting data relating to the network users, where the data is of at least one of the following types: demographic information relating to the network users, Internet Protocol address information relating to the network users, and Internet Protocol traffic information relating to the network users. The collected data is then anonymized, where the anonymizing is performed individually for each type of data. The anonymized data is aggregated to generate an activity-to-demographic mapping representative of network activity associated with particular demographics of the community of network users.