There have been many other examples of posters from this era, on both ebay and offered elsewhere, that have this look (very fresh, a bit shiny, and almost too good to be genuine). It all is a matter of where they were stored or kept.
No fading from sunlight, no foxing, moisture issues or water stains. As the seller stated, the paper was just a little brittle, causing some minor cross fold splits.
It sounds like these were in an attic or upper room of a theater, not exposed to light, moisture, and possibly folded and kept in a box or file cabinet. These copies were obviously also unused, so were extras that were set aside and forgotten about. It's always great to see posters like this be re-discovered and brought to light.
On the flip side, just imagine the number of posters that were left in old theaters and then trashed - being seen as nothing more than paper clutter.