I dropped all of mine at my FLGS for store bucks. It was not much- a few dollars per hardcover. I did get more for some of the supplements and things like a box of 2e spells. Some of the old, rare stuff sells online if you have the time to post on Ebay and such but I think you will not get what you want. You can local sites like Craigslist and maybe a gaming group is playing that edition and looking for some stuff.
I found that my old books were like comics and ball cards in that I played with them and read them so they were not in mint condition.
Depends on the book and the condition of the book. Relatively rare but well-regarded books have values above their original suggested retail value, sometimes markedly so. Over printed and lightly regarded books can have a value that is a fraction of its original value. I've seen used book stores carrying 3e hardbacks at anything from $7.00 to $75.00 depending on the book.
1e and 2e books can go for an even larger range. I've seen good condition well regarded 1e adventure modules go for over $100.00.
Strongly suggest you look at ebay and figure out which books are tending to go pricey at auction.
I would not suggest selling to a FLGS. He is incurring risk when buying product like that and he has to make a margin on resale so you will not be getting a good deal. Ebay the valuable pieces yourself before taking the less valuable components to a FLGS or used book reseller.