I was talking to a friend, explaining how I was persuading a person to do something that will be in my interest.

My friend replied: So you manipulated that person?

At the time I didn't know how to reply. I knew that didn't have the slightest feeling of doing something wrong when I was trying to persuade the other person. Still, I was somehow uneasy with the possibility that I was manipulating a person.

Yesterday I listened to a great podcast by Seth Godin in which he nicely explained that manipulation is only a subcategory of persuasion (marketing).

Persuasion is only manipulation when the addressed person wouldn't have made the decision would he know the same facts that you do. In other words, persuading someone to make a decision that is in his best interest is not manipulation.