I tend to fall in line with Keith's thinking on this subject. We do no one any good by perpetuating stupidity.
That said, it's been my experience that one can point to the obvious deficiencies in a gun (that inanimate object over there that has no feelings) without insulting the seller of the gun (the live person standing beside me who clearly does have feelings). I choose my words carefully, never referencing the choices that have been made. Choices are human. I simply address what is wrong with the gun and how it should be.
As a potential customer, there is nothing offensive about explaining why I am or am not interested in a specific gun. The seller can do what he wants with that info....write me off as an idiot, learn something, scratch his head in wonder....I don't really care. What I won't do is insult him or his choices.
