It's worth mentioning "The Record Collector".
I just discovered them a couple of weeks ago, because they are right near where I live. They have about half a million LPs in stock, most in outstanding condition. It's all worthwhile stuff, not junk like you find in thrift stores. They guarantee satisfaction.. if you order and don't like the record, they will take it back, so it's low risk. It's a phenomenal collection.
It works best if you know what you want, or at least what kind of thing you want. They don't seem keen on letting you just browse, although they will if pressed. It's a little odd.. nothing is labelled on the shelves, but they seem to know exactly where everything is.
They seem to average about $20 for a good album, in excellent condition. I bought a copy of Sheffield Labs "Italian Pleasures" (Michael Newman, classical guitar, direct to disc) for $35, in excellent condition.
They have a website, but no catalog. You have to ask for something specific, or for them to recommend something in a particular genre, etc.
The Record Collector
As a bonus, if you shop in person, you get to listen to your record through their JBL Paragon, which occupies most of the front of the store.