looking at that image, I find it hard to believe that's an original poster
taking into consideration the description 'I have seen several examples of this poster, and they all measure this size, and all are printed in a way that makes it look like the image was blown up from a small image, and that may well be correct.'
I can't understand why anyone would consider selling the poster without more information
that said, I haven't handled one of these personally, so I don't know what the paper is like.
I lean toward Helmut's explanation.
note: Just this week I turned away some material that was either fake or so heavily restored that I could not trust selling them to anyone.
I bought a huge collection of horror-sf and a number of inserts were at the very least, questionable. I don't think you should ever sell anything that you aren't 100% certain of and I'm always surprised when seasoned dealer do.