Speaking as a vinyl consumer, it's really hard to tell. It depends a lot on the physical quality of the vinyl. The best way to get an idea is to actually go to a record fair (I used to cover the one in Ithaca on remote for our radio station, so sweet!) where there are multiple vendors, individual collectors, and thousands of records. Because you will see some albums are ridiculously overpriced, but others will be consistently priced high (it's hard to find a copy of In the Court of the Crimson King for less than $10-12, for example).

Since I know you (MetaChat counts), I'd be completely and totally willing to take your appraisal of a record's quality sight unseen, and make an offer based on that. But I can't really speak for anyone else, because records are a very personal purchase item for me; often loaded with sentimental value. It also depends a lot on the area. When I was in Ithaca, there were about half a dozen places where you could buy or sell used vinyl, and a lot of it was cheap ($2-3 per album, whether for ABBA or ZZ Top, big names or no-names), so you would have a hard time selling most albums for more unless it was rare. People were just emptying their basements and flooding the market.

I don't know your feelings on eBay, but you can always create an auction with the reserve set at the value you think it's worth. Again, I don't think you'll get full price that way because people like to inspect the vinyl. Plus, you don't want to sell them; just find out what they're worth.

Unfortunately, with collector's items of any kind, there's always a problem of "price discovery." It's hard to know what something's worth until you actually sell it (hence the popularity of auctions). Because really, all it's worth is what someone's willing to pay for it. Its value is its value to you; its price is what someone pays for it when you sell it.

This might help you establish a baseline.

I've always compared prices on Gemm.com, and to a lesser extent, eBay (as long as you watch them and see what the final price was). It's nicer/more accurate than dealing directly with a person, because vinyl fiends can be so dishonest. When they give you that "I don't know what this would actually be worth. Ah, what the hell, I'll give you 4 bucks for it", your bullshit detector should go off. I got jacked out of a Fugs record with that technique, and 2 minutes later, I knew I'd been had.
I've also tried to get records appraised, and been told "well, I'll appraise them, but you'll have to sell them to me when I give you the prices". Uh, yeah, that's not an appraisal.
Sounds like you've got a nice collection of stuff, BTW. If I wasn't maxed out on vinyl, I'd make some offers.
Good luck.

it's hard to find a copy of In the Court of the Crimson King for less than $10-12, for example

Really? Yay, me! I've always assumed my old vinyl from the 60s and 70s was worth essentially nil. I've got 3 or 4 hundred of them. Fun to move...

I believe I bought In the Court of the Crimson King fairly recently for less than $5. I wasn't looking that hard, but maybe I just got lucky.

I believe I bought In the Court of the Crimson King fairly recently for less than $5. I wasn't looking that hard, but maybe I just got lucky.

I was worried about getting over-wordy, but what I meant to say is "at the time when I bought it." Time/place are key.