Eil.com

Another great one from the UK. Massive selection in an excellent online database with very reasonable prices and secure ordering.

Vinyl Vendors

I don't think they are quite what they used to be as far as selection, but maybe they just haven't had what I was looking for since they DO still have a TON of stuff. A nice searchable online catalog and good service from a Los Angeles, CA based operation.

Southland CD

Doesn't seem to have a HUGE selection, but usually worth a look. They seem to get some nice 60s/70s items like T-shirts and concert programs that they want obscene (but probably not out of line) prices for. Mostly CDs and tons of promo items here. Located in California.

The Record Mill
The Record Mill on GEMM

This is a great one I just found. They are another UK based seller with a great selection. I'm waiting on an item from them right now :). A little tip: they don't offer secure online ordering on their site, but if you look them up on GEMM (RECORDMILL) or use the second link above, you can use the shopping cart there instead of mailing payment or calling the UK.

Gatefold Records

Another good one out of California. A nice selection in a searchable online database with secure ordering. Great service and great stuff!

GEMM is probably the biggest resource of collectible music on the web. They list the inventories of hundreds of affiliate stores in a searchable database. You can usually order items using GEMM's shopping cart system without ever leaving the site. But you can also find some great sites you'd probably never find any other way. You can also set up a "search agent" to tell you when something fitting your search criteria is added to their database.

Check out Netsounds!

This site is similar to GEMM, often with the same sellers and items being listed. Their site isn't quite as advanced but they're still a contender for finding that highly sought-after item

California Albums

Primarily standard issue 60s/70s/80s vinyl here, but there's really a little of everything and the site can be a joy to look through. All sales are made via email but third party credit card processing is available through Paypal and bidpay. Items are sold in a kind of awkward sort of "auction" but basically you just make an offer and if it's good enough and no one else "bids", it's yours. Not the quickest way to get things, but a fun place to dig. Located in Hollywood

Saturn Records

A little of everything can be found here but not a lot of any one thing. You never know what you'll find here but you'll always find something interesting. I did notice several CD bootlegs here that have become pretty tough to find. Located in Oakland, CA

Secret Sounds

Lots of "standard issue" stuff here as far as mainstream artists, but some of it difficult to find. They claim to specialize in underground and independent releases. Located in Fairfield, CT

Rock of Ages

Always lots of good stuff here coming from the UK. Always worth a look. They offer a lot of magazines and memorabilia in addition to an incredible selection of vinyl/CDs. Inventory is not searchable, but very easy to browse ad many artists have thier own list. Secure online ordering, too.

Vintage Vinyl - Fords, NJ

Seems to be mostly CDs on the site right now, but their retail store is huge and they claim to be in the process of putting it all online. Their database isn't currently very informative (just artist/title/format, no details on most) but they have a neat website, and if they can get their entire inventory on there, it will definitely be one to watch.

Music Imports

This place isn't much for collectible out of print domestic stuff but I wanted to include them because they are a major importer of currently produced stuff from Europe including the colored vinyl releases of Kyuss LPs and various other imported goodies. Located in Ft Lauderdale, FL

CD Express aka Rockhouse - Holland

This one has some neat stuff. Their database isn't very informative but it IS very large. Lots of current European releases, but also some used/collectible goodies.

Ebay

Sooner or later, you end up here...