I don't see how the first sale doctrine applies here.
Based on my reading of the law, it would only apply to lawfully owned and obtained copyrighted material by the first party. In other words, if someone purchase a painting, picture, book, record, tape, cd, dvd, software, etc., the copyright holder can not restrict the lawful owner from transferring their ownership rights (permanently or temporarily) to another individual.
The problem is, these posters were never legally owned by the person or persons that initially gave them away. If the NSS "loaned" the posters to a movie theater, and someone there gave away the poster instead of returning it, that's basically theft. Selling it to another individual does not negate that fact. Fruit from the poisonous tree in a sense. The same rationale would apply to posters given to someone by an employee of NSS who wasn't authorized by the company to transfer ownership.
So unless the poster was purchased or otherwise legally obtained from the original copyright holder (ie: the studio or NSS), any subsequent sales or transfers could still be invalidated.