I read the rules of ToG before my first game and I need 2 clarifications:
1) Is it possible to trade owner markers that are already placed on buildings?
No, but they can be kicked out by playing two markers.
2) According to the rules, it's impossible to "give up a right", so it would mean a player cannot offer to the tower player "I won't fulfil a large order if you go this villa, I'll just take a privilege card"?
All you can trade away is the action in that building. What the player does with the action isn't part of it. You can agree to any terms you like, but they're not enforceable.