Well, nobody asked to hear my theory so of course I'll tell y'all anyway
This isn't to disparage any particular group of eBay sellers, just to point out that we all have our particular obsessions and occasional strange behaviors that result from them... the antique/collectible world is a good case in point. There is a general, unscientific belief among people at large that anything sufficiently old has gotta be "worth something"... which may or may not be true. Even antique dealers, who are in a position to know what's really worth something and what isn't, are not immune to this... you know what I mean if you've ever stepped inside an antique store and seen the "authentic Voyageur era axe handle, axe missing, slight porcupine damage, $50.00"...
The specific sellers I've seen reported here who got all shirty over someone using AS were sellers of antiques and collectibles. They were convinced their item was worth more, to somebody, and they resented the sniper for not giving someone else the chance to beat their bid. Somehow they have gotten the idea that they can make a killing on eBay selling their one-of-a-kind items, which may or may not really be one-of-a-kind...
To put it bluntly, if no one wants your item, its effective market value is ZERO. No doubt there are people in the world who will pay more for it, but they may not have computers, or be bidding on eBay that day.
I don't think any of us are here to rip anybody off. I know I'm willing to pay a fair price for the items I bid on. I just don't want to pay top dollar. I want a deal, not a steal!