I feel some people are being a bit hard on this individual ( I know to whom you refer ).
I appreciate that if you are in a service industry you should always be polite such as good morning Sir and have a nice day etc but although the purchasers are strictly speaking the client, I do think onus is also on them to be polite. I was served by this gentleman today and wished him goodmorning and goodbye and he was friendly back.
What I really detest is people on mobiles expecting to be served. Unless you are able to give undivided attention to the server you should go to the back of the queue.