There are plenty of sites on the internet that you should peruse
before selling the collection. Try Records Collectors Guild and
www.moremusic.co.uk to get some insights what you might get for them.
Used record stores will obviously pay a little less than what they might
be worth, but that's an easy way to offload a large collection. There's
a book and record warehouse here in the DC area that takes in records
in bulk but they don't pay very much. Unless the records are special
or in pristine condition, I can pick them up from anywhere $1 to $3.
If you were close-by I would've loved to see what treasures your father
had in his collection.
All the best.