Once someone has logged in through my authentication site and a session has been started, how can I show them all their data (e.g. an ability to see their past transactions and their user info)?
You need to have some way of linking all of the information about the user - do they have a user id that you can use to query the database to grab all of their information?
You need to have some way of linking the username information with the other data stored about them - e.g. having an auto-incrementing ID field in the users table which is used as a foreign key in the transactions table.