Yeah, I know what you mean. It actually goes both ways, I've had sales people who would not take no for an answer and I've had 'em that just didn't seem that interested in selling anything but Leicas.
Luckily, I've been there enough times, I know who to avoid and try to go directly to the good guys. There are some there who've been there a long time and they're usually better to deal with. That said, the last time I was there, it seemed as though they had gotten rid of the bad apples.
Not making excuses for any of them but I think they all get really tired of unkind customers...Dealing with the public is very difficult work at best. Dealing with the public in a very affluent neighborhood is even more so. I think they have their defenses up.
I always make a point to be extra polite. I try to show than that I'm a serious buyer and I not gonna be an asshole. If I get an irritating sales person I go find another. Just say, "Ok, thanks for your time", walk away and find another sales person. Even if they're in a different part of the store...
Don't know what to say about they guy upstairs in the Darkroom area...but I do buy almost all of my darkroom supplies there.