A bit of a summary to see if I am understanding it correctly.
1. Pre-NSS posters are likely in the public domain as the designs were never protected.
2. NSS posters are likely OK because of the warehouse sales to collectors, thus the first sale docterine applies
3. Post-NSS material is questionable. The theaters never signed agreements with theater owners on disposition, but they did copyright the images.
The NSS material was clearly marked that it must be returned, and theaters had signed agreements. In my mind, it becomes a lawyer battle as to the legality of giving away or selling the post NSS material after its initial use was completed. The interesting case might be the "recalled" posters. Supposedly the studio sent letters out that the material be returned or destroyed. However, nobody to my knowledge has ever produced such a letter, just letters saying "use this new poster instead"
If the studio can retain ownership of a poster, I am reminded of these cases where a muscle car, stolen 40 years ago turns up and is returned to the original owner. The car may have been sold a dozen times in between and fully restored, but it is all considered tainted fruit and the last buyer essentially gets screwed out of both the car and the money.
In my mind it comes down to whether the studio has claims on an item where they didn't provide specific disposition instructions. It would be a different case if a dealer were sellng off a roll, since the studio could show that he material was never used for its intended purpose, and thus was stolen.