Please email with some of the titles which you might consider rare as I might be able to help you.
I have over 11,000 lps at the moment. Most of mine are jazz and R&R from them 50's,60's and early 70's.
Condition is very important as you already know. Some obscure titles or even common titles,if they have no following,do not make in into guides.
Check out the Goldmine books/guide if you can find one at your local bookseller(or Amazon).
There is a great book on picture sleeves but I just cannot remember the title at the moment(try to google some info or try Amazon) Many sleeves from the 80's and early 90's do not seem to be worth that much unfortunately.
Your best bet would be to contact some of the local record stores in your area and ask for their opinion.