For anyone who wants to learn about how to identify and price original movie posters, Google "Lamp." It's a great site with a wealth of information. There are so many fakes out there that you couldn't tell the difference between a fake and an original without measuring it. Many legitimate poster sellers don't even know they're selling a fake.
I have several very rare movie posters that I'll probably be selling soon because I don't have room for them.
It's really nice to have posters of your favorite films hanging in your media room. As an example of a rare poster, the advance "Return of the Jedi" poster was first released with the title "Revenge of The Jedi," until they realized that Jedi don't take revenge. Only a few hundred of these were released and they're highly collectible, and absolutely beautiful. Advance posters with mistakes are also very valuable. Some from famous films are super expensive and usually only available at auction houses.