It is interesting to know that original movie posters were produced by movie studios and distributed to movie theaters to advertise upcoming and current films. They were never meant for sale or to be offered to the public, rather they were supposed to be returned to the distributor or destroyed after the film ran its course, which explains their very limited supply and rare finding.
Of course now there are many reproductions but it is said that original posters from Disney, Dirty Dancing, Silence of the Lams, Alien, Harry Potter, Jurassic Park, Airplane, Scarface, Jaws, Mary Poppins, Star Wars, and the Godfather could gain in value eventually.