The best source is the print catalogs which are updated
pretty often, though I don't know the names of them.With all the info in the net, I don't know of a good
source though you could start with Goldmine magazine
site.
The ideal would be to be able to go to a form, type in
your stuff and get a price. I don't know of any such
source, though there are lists that have been scanned.
That's way to tedious for me to do on screen and so
a print catalogue is preferred.
Also, what to make of the various rating systems of LP
quality? I am not a collector and I don't understand
much of the rating process.