Some insights on this have become a lot clearer to me in the last few years….
The way I would frame this issue is in the context of expressions of personal will, and the impacts of those expressions on both the human agency of others and, ultimately, the good of All.
Inherent to my persuading someone is an impact on their agency - the imposition of my will on theirs. Beyond a specific threshold, this imposition is inherently problematic, regardless of intent or outcome. And what is the threshold? Well, it will be different for different people. Issues like personality disorders, anxiety disorders, PTSD, codependence, differences in social status (including gender status, familial status, positional influence, etc.), and level of ego development (e.g. moral maturity) can all result in a very different threshold for different people…or the same person in different situations. It is therefore incumbent upon me, wherever possible, to understand and appreciate the context and subjective conditions involved. For if I knowingly and willfully impose my will on someone and deprive them of agency, I consider this destructive manipulation.
The challenge here, of course, is in the phrase “wherever possible.” In posting this answer, I could conceivably persuade someone subject to the very vulnerabilities I’ve described to do something “against their will.” But I really can’t know that, can I? So the best I can do in this context is try to state my case and “let the chips fall where they may.” That changes in interpersonal situations, where I can hopefully be more sensitive and perceptive. Even so, some portion of responsibility still rests on a person who is easily persuaded (against their will) to signal their own vulnerability. Likewise, I am also responsible for communicating my own vulnerabilities to others in this regard. But of course both of these situations depend on a fair amount of self-awareness, ability to communicate, and self-control….
Which leads us to intent: what do I intend? If my intentions are sincerely focused on the good of the person I am trying to persuade, issuing from a place of compassion and affection, perhaps this can mitigate some level of manipulation (as imposition of will). Any parent knows this has to be true in regarding their children! But I must also be aware that, to whatever degree possible, if I really do wish for the good of others, then I need to empower them to make the best decisions on their own…that is, to provide good information and - whenever possible - insulate them from persuasion. If mutually agreed to, this helps keep everyone’s agency intact, and (in my observation and experience) enhances efficacy and positive outcomes ten-fold. Indeed, this is also true of parenting.
Lastly we come to outcomes: what is the result? The good of All - that is: the greatest good, for the greatest number, for the greatest duration? Of course this may be desired, but it is never guaranteed - just as the effectiveness of any approach to ethics is not always certain. Which is why wisdom comes into play - including factors of discernment, awareness, insight, timing, etc. And indeed constant practice and discipline; this is how ideal and praxis intersect.
So for shorthand, we could use the formula: compassionate intentions + situational awareness + self-awareness + appreciation of cultural/power dynamics + situationally adaptive skillfulness + predictive efficacy = non-manipulative persuasion.
As you can see, there is quite a complex balancing act here. And honestly I believe many cultural value systems promote this balancing act in the normal course of human interaction - in other words, it is tacitly implicit, and transmitted by example. Unfortunately, some cultures (and indeed some individuals) lack the emotional intelligence, language and perceptive ability to recognize, internalize or actualize such a formula. And such challenges can then be exacerbated by commercialism, consumerism, individualism, materialism, selfishness, family abuse, workplace stress, social injustice and so forth. For those cultures where tacit understanding and transmission are absent…or for individuals and relationships that have yet to fully invest in such compassionate dynamics…well, then it would have to become a more conscious enterprise. The challenge then becomes that, for certain folks who routinely capitalize on manipulative persuasion (i.e. profit-driven marketing and advertising, religious zealots, political ideologues, etc.), there is tremendous resistance to such awareness, sensitivity and caring about the agency of others.
My 2 cents.