ok, since Rich can't self promote, I'll do it.
Rich's movieposterbid is an honest site.
You get the better deals and don't have to deal with fakes at the better auction houses. I personally shop at the following (not exclusively, but probably 70%)
www.emovieposter.com - My first go to source. Honest and usually able to find good prices. Sometimes it does get crazy.
https://movieposters.ha.com - usually a step up, but at higher winning bids. Has a buyers premium.
http://www.movieposterbid.com - Rich's site. He has been quiet for a bit, but recently started having auctions again.
www.movieposterexchange.com - Phillip's site is currently down for remodeling but should be back up in a couple of weeks. Usually lower priced stuff at great winning bids.
You need to pay attention to auctions that allow 'sniping' (robot programs that make last second bids) and ones that have extended bidding which extend the time when you make a late bid. Develop a strategy.
I mostly stay away from eBay these days. Been burned a few times with misrepresentation and poor packaging.
Well, I seriously doubt people will post their best places here... ahaha, you know poster collecting is a highly competitive hobby
Another factor to take into consideration is shipping cost, depending of your location. If you leave in EU, then you are better off buying in EU; same for US... If outside US or EU, go for auctions/companies offering an hold period (that is keeping posters in their warehouse for you until you have a good quantity of posters). The plus side, your shipping cost is greatly reduced (for me, as little as 3 USD per poster when shipped combined), the down side you end buying stuff you may not really need.
I shop there, like every body else:
www.emovieposter.comhttp://www.movieposterbid.com Agree Rich is a top guy. Emovie is great but as said above, things can go crazy, make sure you check condition of items there as there can be some surprises.
I never bought from HA, but I am following auctions, posters are indeed higher quality but the 20 USD premium is a killer.
I avoid Ebay unless it is a seller from here, or with good history, or it is something I really need at great prices... main reason is poor shipping packages resulting in stuff being damaged.
But there are indeed some shops online where you can find posters are relatively good prices. Most shops have high prices, much higher than emovie history prices for example... but :
a) Don't despair, with persistence, you will find good shop;
b) You can always ask for a discounted price.
To find these, use Google Search and a text document (Notepad, MS Word). Search for specific terms and posters in Google (use different languages or do search in restricted local DNS), and if you find a shop, add it in your document (under region) and put link to posters of interest with prices... do the same for other shops and similar/other posters... it can take time but once you are used doing it, it is a fast process... and the good thing with this is that:
a) You can see with time if prices has changed;
b) You can easily compare shops to know which one is cheap, which one isn't;
c) You will have a nice wishlist with direct link to shop.
Last but not least, plenty of resources here, search for "sellers"...
Good luck