I don’t know how true this is. Some of the best sales people I’ve known weren’t particularly good looking.
Most doctors are average looking.
Same thing with lawyers.
I've noticed that too.
Of course, like most electrical engineers, I look a bit like Pierce Brosnan from some angles, and quite a lot like Ben Affleck from the others. Even so, the other EEs gave me the nickname "Ugly" when I was in college.
The art of manipulation has everything to do with one thing--manipulation. In fact the better looking a salesperson the less a person identifies with them It depends somewhat on the product.
All purchasing however is based on trust. Sometimes being too attractive may actually be counteractive to the manipulation. If someone feels they can trust you, you can sell them anything. If one is a normal person that is charming or not unattractive they can use incredible manipulation to fool you as we think of them as like us and normal. That is the door that opens the relationship.
A good example of this is the okay looking single brunette next door to you and your wife. The wife isn't threatened by her and sort of gives you and her a wide berth. If she was super hot the wife would be all over any interaction even just a small sigh unrelated while the hot woman is in the same sphere.
The boring brunette gets more access to you for manipulation and also she is also seen by you as less of a threat. Next thing you know girlfriend's got you by the small ones and wifey comes home to find you in bed. haha. Of course I know this would never happen to any FReepers. But you get my drift. Average looking people can be excellent stealth manipulators because your guard is down.
Some plainness is an asset to credibility in positions that carry notable responsibility, because people are less likely to wonder if the person got where he or she was by his or her looks.