Having been on "both sides of the table" I can say that I have seen every one of the things described in that post, either firsthand or secondhand.
I still have some of the sleeves, top loaders, and cases that have my price stickers on them - from the shows I did 27 years ago. It looks like I had all the right ideas back then. I used to have 8-12 year old kids come up to my table asking to trade. I didn't do the flinch. I showed them the cards I wanted, the cards I could sell, and the rest that would go in the commons box, never to be heard from again. They appreciated the fact that I'd look to make the trade work rather than say "No kid, I ain't trading today." like so many of the other delaers did. (I actually had many of the other dealers coming over to my table to see why there were so many people around it throughout the day. I was mobbed by kids wanting to trade the duplicates off.)
But, when trying to sell stuff to dealers, I saw the flinch. I saw all the rude behaviors. Oh, and I usually had my dad there (at least part of the time) and made sure he was there at the table if I left the table. Never was my table unattended. I also wound up being the last person out of the hall. Mainly because I was still working deals (sometimes with other dealers), but because I didn't want to lose a sale by not being there.
And why on earth would you take something to a show you wouldn't sell? The only thing I took that I wouldn't sell was the plastic shelf that I'd set up behind the table to display wax boxes (upper shelves) and store my take home stuff (bottom shelf). Anything else there was for sale. And if someone had offered me enough, the shelf could have been theirs too! :)
Looks like it all boils down to just practicing common courtesy. Treat the customer (and treat the dealer) the way you'd want to be treated. Pretty simple.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).