The sellers for the most part appear to be disclosing their listed posters as reproductions but the disclosures are sometimes stated at the bottom of listings or in a paragraph or more of text. At least the sellers are disclosing this information although it is doubtful they have the rights to reproduce the poster artwork and sell it.
I had unthinkingly bought a reproduction of a "True Grit" poster a few years ago because I could never afford to buy an authentic 1969 poster in decent condition. The reproduction is nicely printed on heavyweight, unmarked, unfolded bond paper. Not bad for $12, I had decided.

However, it wasn't until recently that I really looked at this poster--and saw one sign of a reproduction taken from an actual (I assume) poster: fold marks. This is a low-res scan of most of my poster, the green arrows inserted to show the folds that were apparently in the original poster:
Dee