I am buying point to point train ticket from travel agent for journey from Brig to Milano and then Milano to Venice. For each ticket he is adding sector fee of Euro 8. What is this sector fee? I also purchased ticket from Trenitalia.com from Venice to Rome but it did not charge(does not show on ticket) any sector fee. Is it something that is charged by agents only or applicable on travel from one country to another?
I think your travel agent is just finding ways to increase his profit. I NEVER use travel agents. It's not hard to book directly from the rail companies.
The "sector fee" is being charged by your travel agent; it is not charged by Italian Rail (Trenitalia), as you discovered. So, buy your tickets from Trenitalia, not from an agent. Here's a detailed tutorial on how to do it, from the invaluable Ron In Rome: http://tinyurl.com/c4t693b