You just stumbled on a gold mine! Those are amazing!
Looks like one of them has been out there a little as evidenced here.
It was also referenced by Steve Sansweet in this book.
As well as in an issue of Starlog:
Some posters have never even been released, and that makes them worth even more. "The Star Wars pre-production poster we have for sale has generated a lot of interest," remarks Jack Wolffers of Richard Wolffers Auctions. Inc. The rare Star Wars poster is a hot item because it features an early advertising slogan which was discarded in favor of the now famous "In a galaxy far, far away..." One wonders if this proposed slogan, "First, Buck Rogers. Then, Flash Gordon. Now, Luke Skywalker," would have been nearly as effective.