In sales you necessarily paint your products in the best possible light- demonstrating the benefits and ignoring/discounting the negatives. Isn't doing this inherently deceptive? Car ads will say "best fuel efficiency in it's class". Do they really have numbers to back this kind of claim up? Or do they just figure people won't check because it takes too much work, and that if they repeat the claim enough times people will believe it anyway?
How can any business claim they have the "best" product, when "best is so subjective? Value itself, the stuff money comes from, is completely subjective and can seemingly be inflated/deflated at will (based on how you frame the wording/imagery in the ad.) Do we just ignore the fact that we may frame things incorrectly in an effort to get things sold? It's no wonder most of the population views "salesmen" in such a negative light.
And what about using psychology? Quickly you find out if you're in business that human beings respond to certain emotional cues. We avoid pain and seek pleasure. We have things like vanity and superiority that drive us. Ads almost seem to get to the point where they become the same, because everyone responds to the same emotional "tricks" again and again. Maybe the person who knows the most just has the power in this world, and we should accept that people won't know what we're doing. (I personally have become much more aware how society "manipulates" me through advertising since I started studing what people want.)
I guess the only thing you can really do is be as accurate and as truthful as you can. But it seems to me no matter what you do you really either end up lying or manipulating people when you try to sell something. Maybe that's just the way it is.