I'm in sales (not audio stuff) and have had a fair bit of training. Never make a disparaging remark about what a customer has, just extoll the virtues of what you might be selling. As for how to treat a customer, I've tested automobile sales people, as I did that job for a year or so...successfully.
I would show up in a friend's beat up pickup truck wearing tattered clothes, couple days beard, unkempt appearance "work dirty." Wander onto the lot looking at what I was interested in an take note of how I'm greeted and treated. Show up a few days later all clean shaven, carefully groomed and wearing my Hart, Schaffner Marx suit. Approach the same salesperson and see if they treat me any differently. Most never even recognize me. What a revelation. Most failed, those who passed had a good chance of selling me a car.