Mel
I think the problem is you are a very atypical buyer! You do your homework and you know how to tell fakes from reals, and you would likely buy from the minty white sellers if you were sure that particular poster was real.
But most collectors don't have the time or dedication to research everything out, so they go on reputation, and that is where "perception" comes in. If one seller sells fakes side by side with reals, and another sellers has a zillion lawsuits going against them, and a third seller has a great reputation as a "fake-free zone", then many buyers will, as fallenangel says (and thanks!) pay more to that seller than to the first two.
As to your comments on terrible eBay packing, I am a major collector of travel posters, and in the past 60 days I bought over 250 of them from eBay sellers that arrived in the most motley collection of tubes possible. That is going to be my topic for next week!
Bruce