I buy from folks that I have a relationship with, for the most part. Whether they are considered dealers, collectors, or some mix of the two is irrelevant. As Ves said, if someone has a poster I want, and if we can reach a mutually acceptable price, then I'm a customer. Even with ebay auctions, I deal with sellers that I know about 95% of the time. I would rather pay a bit more to deal with someone I know than go for the "great buy" which turns out to be not such a great buy when it gets torn up due to shitty packing and shipping. If you factor in all the crap you end up with buying just from ebay dealers that you don't know, then all of the sudden I bet those great deals don't look so great when you spread the cost of the useless crap among the few really good things you get.
And you're right, Mel, his is an ad-free forum and we can speak our mind here without worrying about advertising. At the same time, I think it's important to acknowledge the good dealers out there who give fair prices. How about Posteropolis? I've bought some great posters at great prices from Dave. Phil from Cinemarts, Ves from the Art of Movie Posters, Rich and Movie Poster Bid, Bruce at emovieposter, Colin at ckmac, for a time, Steve Zammer from dsonesheets before he went awol, Erich, Carson, Dale, Dave at cinemasterpieces, kinoart, posteroid, ed king, I could go on and on with a list like this. These are all folks that I have bought from, and from whom I have received great products at great prices with great service. I'm beholden to them not because they advertise, which they don't, but because they are all some of the best in the business. I trust them, their products, their knowledge and their ability to ship something safely and securely. Tell me how many auction-only types on ebay you can say that about?
Now, don't expect to jump in and get the buy of the century when you're new to this hobby. These are relationships I've built over the past 6 or so years. As much as I trust them, they trust me to follow through with a transaction and to not jerk them around with asking for a million pictures and never buying anything. It's a two way street. I'm sure there are a number of buyers on this board who are not liked by every dealer out there, because of bad attitude, bad negotiating style, or because they're just to damned cheap to be bothered with.
Remember, for all the anti-dealer types out there, one day one of those dealers may have your "unicorn" for sale, and you damned well better hope you haven't made an ass of yourself before then with that dealer!