provenance in movie posters is as T says, not really much of an asset.
Art yes, other unique one-of-a-kind items yes. A car that used to belong to Elvis.. yes..
in posters, other than a truly unique piece (like say "THis Metropolis 3 sheet belonged to Leonardo DeCaprio") it means nothing as he was not the first owner. It doesn't add to it's cachet
here's an example
My friend has an insert for Double Indemnity that (supposedly) was owned by Stanwyck.
I need this poster and will trade him for it. But the possible Stanwyck ownership, which really can't be traced, means nothing to me and it means nothing to his price.
It isn't signed, it comes with no letter, there are no pics showing it in her home, etc
if a set of James Bond posters from Dr No to YOLT came from Connery's collection and is sold at an auction that can include a letter stating so, that could have an added value to someone, but fo rthe most part, who owned a poster - especially a poster for which many copies exist - it's nothing