Well, you have a choice, you either sell them individually yourself or you sell the whole lot to a used record store.
Yourself = You get the whole retail value but it takes work as you will need to look them up individually to find their value and there is very little on
ebay, although that is where to start. Be careful though, some records can have a couple dozens version what with the US pressing, French pressing, english or german or japanese pressings, some of which can have soiught after covers.
Then you also need to learn how to judge of their quality... Are they new, like new, excellent or whatever?
The biggest problem is that even a book like this one
Amazon.com: Goldmine Record Album Price Guide (9781440203732): Martin Popoff: Books doesn't necessarily have the exact album you have.
The last little thing to know is that it will take a while to sell everything. It's definitely a long term project.
Record Store = They pay about 1/3 the retail value if they know what they are doing, meaning if it is a collector's store. Most of them are today because old vinyl is bought mainly by collectors. But it is true that there are quite a few people buying vinyl (new or old) because of the better sound over CDs.
I sold about 1500 records in the last couple years before moving and the prices ranged from $3 to $400. I used the book linked above, a bunch of different vinyl collector's sites (Google is your friend...) and an old friend whose store I bought an awful lot of records from over some 20+ years to price them. I won't mention how much I made because it doesn't matter. It really depends on what you have.
Now, here is my priciest sale:
File:Virgin Killer.jpg - Wikipedia, the free encyclopedia
I sold a few copies of this thing which I had gotten over the years in Europe because I knew the german and french cover which was banned in the US was very appealing to american perverts... LOL.
Have fun.