If you can't find an individual person for them, you might contact Amoeba on Haight St and see if they'd be interested and could send a buyer to you to check them out....
I second this. Contact Amoeba. Otherwise, try Acoustic Sounds in Salina, KS. They buy collections. Whether they want to bother with 600 LPs might depend on what you have: I cannot speak for them. I know they are interested in my collection (when I die), but I have 5000 LPs.