The way I use the term, manipulation is influence that is improper either due to the methods used, or the social context in which they are being used. Our society (and I suspect any complex society) will have very complicated norms about what kinds of influence are legitimate in what circumstances. For instance, lying to induce somebody to do something they wouldn't do if you told them the truth is improper manipulation in most circumstances.

A lot of our norms are area-specific. For instance, in law, there's a very well developed set of principles about precisely what constitutes misrepresentation or undue influence -- in which case the contract can be void.

I suspect, in general, there will not be good reasons for putting the demarcation at one point over another: I imagine that social processes will drive norms towards at least a local cost-benefit maximum. And if the cost-benefit landscape is smooth, those local maxima will be places where there landscape is locally flat, implying that there's negligible harm if you move the equilibrium slightly.