I think there are several reasons why some types of commercial posters are becoming collectible and more valuable.
the most obvious reasons are that in the 1960s & 70s, for many people, commercial posters are the only things they knew were available to buy.
Surf lovers didn't have Endless Summer one sheets on their walls, they had the Personality Posters 30x40 silk screen posters up, like this one I sold last year
http://old.movieposterbid.com/itemdetl.asp?id=107822
while the owner may have a nostalgiac feeling for the movie poster if he saw the film in the theatre, he is much more likely to have a nostalgiac feeling for the commercial poster (although, this is a rare case where a commercial poster was also used theatrically at some theatres and drive-ins, probably due to it being available via catalog).
The great Barbarella poster, we all had the hots for Jane and that poster image is not available in any theatrical paper that I'm aware of
How many people had a Pink Floyd poster for the Wall? oops.. I forgot to add, but was it the movie poster or a store-bought poster? or did it come with the record album???
Right now I have a hug load of comic book and sports commercial posters. The bulk of the comics posters are 80s-on, but the sports posters are 1960s stuff - a great period for sports posters. I think they will sell well because these are what we had in our homes, on our walls, in our basements, in our garages, in the clubhouses.
Listen, I've had movie posters since I was 10, but I was involved in the hobby.
99% of the other kids had store bought posters, including this one
and this one
