I dared. It sounds like some venture capitalist wrote a bad hippie-ish novel, published it via a print-on-demand method, and distributed copies everywhere, sometimes for free. Reviews seem to indicate it’s a bomb - nearly literally in one case.
Two things that haven’t been mentioned:
Ann Rule rules the true crime genre in used book sales.
Chicken Soup for the Soul and all its imitators.
Danielle Steel (especially the Klone and I) Nora Roberts/JD Robb, Stephen King, Scott Turow, John Grisham,
40 year old pamphlets on filet crochet.
Crossword and/or Scrabble dictionaries.
Poker and/or Blackjack strategy guides.
Biographies of Michael Jackson.
Possibly the funniest remark I’ve ever seen.
Thank you!
I’ve seen quite a few Stephen King books at the sales I’ve been to, but they tend to be his lesser works - like The Tommyknockers, Needful Things, Insomnia, etc. rather than the classics.
Chicken Soup For The_______Soul. And various volumes from Womans Day Encyclopedia of Cookery, which every household seemed to have collected during the 60’s. (We had a set and I read them from cover to cover, very interesting and useful information.)
The local Goodwill had and still has an impressive collection of Danielle Steele and Left Behind books. I’v enever seen such a collection of complete crap in my life. Yes, they had Wordperfect for Dummies 
I wouldn’t pay a nickel for 95% of the stuff there.
When we moved out of my house into a smaller townhome I decide to ditch almost all my books. I had a huge collection of books that I had going back to 7th grade. My wife doesn’t read em. My kids aren’t interested in em. So…time to ditch them all except for the few I know I will want to read again.
I donated them all to good will. They actually set up a new temporary section for my books and put them all in there. Many many shelves 
I went back in 3 days later and, except for about 4-5 books…THEY WERE ALL GONE! Obviously a hunger for different kinds of books at GoodWill Cashier said they were flying out like clockwork past 3 days.
Oh man…I’ve sorted god knows how many books for used bookstores, Friends of the Library, etc. Authors not already on the list: Sharyn McCrumb, Anne Rivers Siddons. Also (at least here in DC): Terry McMillan, Maya Angelou, Zora Neale Hurston.
I agree with Future Shock, Megatrends, and Passages from the list link given by The Lovely Margo Lane.
I’ve always thought that …And Ladies of the Club is designed purposely to fuck up all attempts to create a neatly alphabetized fiction shelf. There has been at least one copy at every used bookstore I’ve worked at, it never sells, and it completely screws up an entire shelf in your already-crammed S section.
Kitty Kelly. I think it’s a requirement that all library sales have at least five copies of “The Royals” on their tables.
Wow, The Royals. Man, that book was bigger than Jesus when it was first released.
One more that needs to be added: Twilight
And one more that will be a booksale staple in the future: The Secret
At first, I read that as “Unless they just burn all the undead books…”. Which would seem rather apt, for some of them.
There’s usually at least one two-volume book that only has one volume.
And yearbooks/almanacs, usually at least 15 years old (e.g. The Year in Music 1993).
There’s often a volume of World Book encyclopedias as well. No idea why it’s usually World Book.
Clan of the Cave Bear (or Jean Auel, generally) used to be dependably present at thrift stores. Has it finally died out?
I can’t believe that it’s gone this far without anyone mentioning V. C. Andrews. Her sleazy dramas take up at least 30% of books at my local flea market.
While I mentoned Mary Higgins Clark in the OP, her daughter Carol Higgins Clark and ex-daughter-in-law Mary Jane Clark are well represented, too.
I don’t know, but they certainly go hand-in-hand with James Michener books in the category “Books my grandma would read”.
Now that she finally wrote another book in the series, we’ve sold out of her used stuff. People wanted to catch up on the old ones before reading the new one.
Long, long ago, I took a Shakespeare course from A Professor who spent every class paging through his yellowed notebook & droning on about the Freudian/Oedipal story behind every damn thing that Shakespeare wrote. Hamlet? Well, there’s a hint. But he found the same pseudo-Freudian theme in every play & poem. The Histories? Why, England was Mom & everybody wanted to have Her!
Did he give any clue that all classes of people enjoyed those plays & still can? Nope. Any hint of the music of Will’s language? Nope. Just his bored/boring recitation of the one theme he’d developed so long ago–before he dried up entirely.
For years, every used book store in Houston had at least one copy of The Professor’s one slim volume of verse. I never opened one because I figured he had no language skills. Apparently, most of the people who’d acquired that volume agreed with me…
I see most of these at the used book venues that I frequent and will add Guinness Book of World Records any year to the list.
Everything by Kindergarten author Robert Fulghum.