I took my old book collection to my local used book store, they bought what they wanted for credit (1/4 of cover price) and donated the rest to the library for me. This is a small town, though YMMV.
My daughter is assistant manager at a local used book store, and she frequently buys books. I asked her advice on this, and added my own insights as someone who frequently haunts used book stores.
Her store has a monthly cash buying budget, though if they need the books to improve their stock, they’re more likely to make you a good deal. They will always take books that they need in trade. They will always look at books, no matter how big the collection, but if you bring in a lot of books, it might take some time for the staff to go through them. Such as, it might take DAYS, as the main book buyer might not be working that day, and s/he is really the one who will approve most or all of the purchase. In the store that my daughter works at, the owner or his daughter can go over the buying budget, but nobody else can. So, sometimes it pays to wait.
Her store gives more store credit than cash for the same book. For instance, a book that might bring you $1 in cash might bring you $1.25 in store credit. I’m completely making these numbers up, she’s out of the house now. But you get the idea.
If the books are romances, westerns, or “men’s adventure/action” books, don’t expect to be able to sell many of them, unless they’re rare. In fact, many used book stores won’t take these titles at all, except in trade for others of the same genre.
If the books are severely damaged, chances are that you won’t be able to sell them. By severe damage, I mean major water marks, pages falling out, etc. IF the book is exceedingly rare and sought-after, you MIGHT be able to sell a copy in bad condition as a “reading copy”, but don’t count on it.
Sorting the books according to category would be most helpful. If you REALLY want to be nice, sort according to hardback/trade/paperback within category.
Ol’Gaffer is quite right. Most used book stores have more than enough “bestseller” authors, though sometimes they do run low. Don’t expect to get much money/credit for such an author, unless you happen to have a first edition of the author’s first big seller.
If you KNOW that you have a book or a few books that might command a high price, then you might leave it out of the group box, and try eBay, Amazon, or other auctions. I know that on eBay, at least, quite a few people group books of a similar category or author, and sell them in lots. The titles, authors, and usually condition are listed for each book in the lot. You might find it profitable to break your books into lots of 10 each, if you think that selling them this way would work. This is chancy, though, and rather time consuming.
In science fiction conventions, there are areas known as the “hucksters’ tables”, where all sorts of vendors peddle their wares. If you have a lot of science fiction/fantasy, it might be worth your while to rent a table. I don’t know if other genre conventions have such things, though I suspect that they might. I have no idea whether you would find it profitable to rent a table at a flea market. However, as a BUYER, I am far more likely to browse a table of books that are sorted by category, and preferably by author within the categories.
You might have to visit several used book stores and use several of the methods listed in this thread to find homes for all your books. Good luck!
Then thre is Goodwill or the Salvation Army
Most of my used books end up not being worth very much, but if you think some of yours might be, a good place to check it out is
http://dogbert.abebooks.com/servlet/BookSearchPL
Most of the books are listed by dealers and they usually give pub date, edition, publisher, and detailed comments on condition. You can also list books for sale there. The books are from all over the english-speaking world.
ever considered bookcrossing before selling off the books?