There is nothing more appropriate and more pleasing for a customer than when sales assistants and other shop staff treat them with respect. It should also be noted that it is because of the continued desire of customers to shop in their stores which keeps sales assistants employed.
Sometimes no doubt because they are so dedicated to their work and focussed on the task at hand these staff members forget that customers are the most important people in the store.
Staff can sometimes go so far as failing to acknowledge customers when they make a purchase, either ignoring them completely or failing to engage at all, preferring instead to discuss their credit card bills or finishing time with their nearby work colleague. At other times staff members will walk directly in front of a customer while they are browsing store merchandise either blocking them completely or pushing through without consideration or offering an apology.
Please remember that customers are a necessary evil in this world of consumerism and retail therapy, especially if you are in the employ of one of these sales establishments. Therefore at least a base level of civility in dealing with these valued people would certainly not go astray. Perhaps a smile, a kind word or at the very least a passing acknowledgement is most certainly in order.