A computer service associated with an electronic catalog of items enables users to explicitly elect to expose their respective purchase histories to other users. When a first user accesses an item in the electronic catalog, the service may notify the first user of one or more other users who have purchased...http://www.google.com/patents/US7254552?utm_source=gb-gplus-sharePatent US7254552 - Notification service for assisting users in selecting items from an electronic catalog
