Your users or salesmen table should have an id column that is an auto incrementing value. This is the id that should be used as the unique identifier to limit access of the results to a particular salesman. When the salesperson is registered then the id will be associated with their record. Then whenever they login you will be storing this id in a session variable. On any of the pages where you are viewing details that are specific to a particular person you would filter this recordset by the id of the currently logged in person.
You would also use this id when inserting sales info, you would insert the id into the sales record so that it is associated with the sales person who made the sale. As for adding in anything form eCart you would only do this for the commerce portion of the site where you would like to be able to add in items to a cart then checkout. I don't think eCart is going to do anything for you in regards to showing you the most popular items. You can make a reporting page like this but it would be a custom query to check to see the most sold items.