Learn to Just Say Yes

Many people in business, customer service or other people-facing roles do not know how to say yes.  I don't mean to accept something, I literally mean that they seemingly do not know the word "yes."  They say "yip", "yup", "yeah", "uhuh", "okay" and a few other similar responses, but for some reason they never learned, or more likely (all kidding aside) they didn't develop the habit of saying the proper word "yes" when doing business. 

What's the difference?