I know of a few online retailers that specialise in out-of-print (OOP), second-hand, and original editions of RPG books and accessories. Let's make a definitive list of them!
eBay has always been my source.
Noble Knight Games carries OOP stuff, but the prices are premium, and things are not always in-stock.
For used used books I usually start at abebooks.com which is an aggregate collection of used booksellers across the country.
Here's the results for 'Dungeons & Dragons' first edition published by TSR before 1990.
And to a lesser extent, Classifieds at Dragonsfoot.org.
I bought them there because it was the cheapest place I could find my books. Surprisingly they arrived sooner than when I ordered some other books to Amazon. That was some time ago and I was at Mexico, but I keep this link as my favorite place for used books.
Troll and Toad is a second-hand and first-hand dealer that carries out-of-print and recent products.
If you have any used book shops nearby give them a try. I've actually found quite a few original D&D and AD&D accessories at some in VA and MD.
Within Ebay look for the CrazyEgor store. He specializes in want lists, so you can send him titles you're looking for. He'll keep your contact info if he doesn't have it on hand now, and let you know when it's available. He knows what his stuff is worth.
When you use ABEBOOKS.Com or Bookfinder.Com it pays to search loosely, as not all those offering second hand material bother to enter the entire author name, or check the spelling on titles. In "classic" format [look below the top screen], it will rank the books it finds in ascending order by price.
It is a French store, but they have a lot of English material as well.
They have plenty of second hand books and they are also specialized in massive discount for discontinued games or bankrupted editors. (Lot's of brand new D&D3.5 and 4 books around 10€ for example). They are also dealing CCG and boardgame.
Very nice store for filling holes in collections and to find some second hand rarities.
I don't know the foreign delivery prices and conditions. Since there is an English version of the site I guess they are shipping abroad.
Amazon, stil has some 3.0/3.5 d&d material.
RPG Geek is an RPG database, and the user features are mostly about listing your collection, but it's also useful for coordinating sales and trades. People can mark items in their collection for sale or trade, and you can add items to your "want to buy" or "want for trade" lists.
The ASGI RPG Store (www.asgi.co.uk) has a reasonable stock of items and I've bought a few things from there (1st Ed) with no problems - usually pretty quick delivery and response to e-mails.
Prices slightly higher than individual seller eBay auctions but lower than most of the eBay buy it nows, plus you don't have to wait for someone to list a particular item - it's just from stock.
Sentry Box is pretty good. Remember, the prices on amazon are WRONG.