Thanks for the replies. I agree @frissdiegurke that it is a social network feature. Essentially I am looking for another way of adding reputation. I.e. the current one shows how helpful, accurate, ... a person is. Now suppose that this forum is also a media for people to buy stuff. Now, we also need to rate sellers based on their quality of service.
Is it still doable by a plugin, or do I need to change the core?