Thank you for letting me be a part of this. I excited to a part of this wonderful community. I'm new to posters, just bought my first poster, a Star Wars 1977 One Sheet. I have been a big movie buff and I wish to own and display posters of some of my most favourite movies in my home. Posters of E.T., Saturday Night Fever, The Graduate, Lolita, The Lion King, Taxi Driver are some of the ones I am looking to buy next.
Welcome, posterchild. Welcome to the madness.
The stuff you mention has all been 'discovered' and thus will sell for some $$. What you need to be careful about is doing your homework and making sure what you buy is genuine. Much of what you mention has bootleg copies floating around. There are sites which describe the well known bootlegs and how to avoid them.
The best way when starting out is to work with the more reputable auction sites. emovieposter, Heritage Auctions, Movieposterexchange and movieposterbid are all reputable sites which will periodically auction these titles. Again, do your homework. Bothe emovieposter and Heritage have sold archives where you can research a ballpark of what your potential purchase might be going for. This avoids going into a bidding spree and paying double what it might sell for a few weeks later. Of course, the final decision is yours, and a poster is worth what you paid (to you).
The last chunk of advice is to exercise patience. Wait for the right item at the right price. You may lose a few, or pay too much (likely both) while you learn the ropes, but after a bit you gain a feeling for the market, and also get into the joy of the hunt.
Good hunting