Unfortunately, art is subjective. It's not like, say, shoddy repair work where the buyer ends up finding out LATER they were ripped off. They see what they're getting, and are buying it because they like what they see. Often folks buying the work haven't any artistic leanings themselves, hence they can't really notice things like poor anatomy or perspective -- so to them it doesn't really matter. You can't control what people like. Telling them that one person's art was made with better skill does not change which aesthetic they prefer.