HJ, I actually agree with virtually everything you said. But I don't think this justifies any business being rude a-holes.
For example, I didn't say that the supply place is going to give a retail customer YOUR price. I did say that they can choose to come close to or beat the known retail prices, like the Big Box price, and they still aren't going to be anywhere near what they will sell you the product for. The Big Box price is out there; the online price is out there, and there's no reason for them not to be willing to sell retail near that to a buying-it-today cash customer.
The customer you describe, on the other hand, is a total idiot. Did SHE pick the product up and bring it to the job site? Did SHE have the expertise to determine whether the product would be installable in her designated space? Did she bring it upstairs to the bathroom and figure out how to get it in the door? Did she put it on her tab (or pay cash for it) long before it was actually installed and thus affect her cash flow? As far as I am concerned, all those factors can and should be reflected in the price at which you sell it to her. You are NEVER representing to the customer that the price you are charging her is the exact price you paid for it. You are charging her overhead and profit and a DELIVERED price. Which is totally fair.
I don't begrudge people who try to negotiate a lower price. The vast majority of the time, I just tell them, "No." Nobody has a right to use you or use me. I think explaining to the idiot customer how the system works: "This is what I am selling the tub to you for, and here's why," is generally good enough. Most people will take "No" for an answer with an appropriate explanation, and for the few who don't, you don't want repeat business from them in the future anyway. I sure don't, and I don't worry about the few nitwits that have walked away and hired someone else. I try to be a good person, so I don't take pleasure in knowing that when I checked on their matter later, they...um.. got the quality of result that they paid for.
I don't think, however, that treating a regular customer rudely on the first encounter, or assuming that they are an idiot and treating them as such, is good business. I do think that treating professional customers as a priority is appropriate. If a civilian is asking a million questions and there is a trade customer in the store and nobody to help him, all one has to do is say, "Give me a moment to deal with this pro customer quickly, and I will come back and answer all of your questions."
I tended bar back in the day, and I've been a partner in hospitality businesses. All bartenders end up with a few folks every night who are really needy and, left to their devices, will monopolize the bartender's time and keep him from interacting the way he needs to with his other customers. That's bad for the other customers and thus is bad for the bartender and the owner. The problem is worse for female bartenders. I have taught everyone I have trained a simple solution: you just cut them off and say, "Give me a sec. Be right back." And you WALK AWAY and do what you have to do, and return to that person in the normal flow. Works perfectly, and the customer is always cool with it if you do it right. This works in every business that has a counter at which you deal with people. (You also immediately acknowledge -- "Hey, Guys! Be right with you" -- anybody who comes in the moment they come in, even if you're in mid-sentence with someone. That immediate acknowledgement actually makes the wait seem shorter for those who walk in.)
All bartenders also have horror stories about customers, and all bartenders can make an initial sizeup of a customer that will be accurate about half the time. They think it's more than 50 percent accurate, but in truth it's not. They just make it a self-fulfilling prophecy. By treating all customers very well -- as if they're the one that's gonna surprise me -- I got surprised a lot, and I made a lot more money than others did.
I don't think that any plumbing supply needs to treat me like crap just because I don't have a Carhartt jacket on. But if they want to do that, that's their perogative, and I will go elsewhere because I sure as hell am not going to give them my money. And they can bleed to death and die serving the diminishing market in which someone just calls their plumber and asks him to furnish and install a toilet, while others learn to do it themselves and go buy the product somewhere else, or buy it through a design/build showroom who is going to sell them the product directly and hire their captive plumber (or, worse, non-plumber) to install it.