Respect and understanding is a two way street...but requires one party to begin building that street. You would have gotten much sympathy if you showed nothing but respect and patience throughout your interaction with this person...especially if he only returned contempt in exchange for respect.
But as a human being able to see both sides of arguments, I can sympathize with your view that vendors are there to serve you as you are the customer and your patronage is what lets them exist...but I can also sympathize with vendors who deserve a modicum of respect, patience and understanding when it comes to things they do not control amd when it comes to any social short coming they may possess.
Or does it strike you that only the customer is permitted to have social short comings & only they are allowed human emotional reactions to what is being said to them or AT them?