For individual books, there was the 'you didn't like it when you read it the first time, why are you keeping it?' A few in that class got spared because I might want to reference them for rants on why they are so awful; all of my Benford and Mote in God's Eye. Some I had bounced off of trying to read, so unless they were a classic I really should read sometime (sadly, I have a lot of Delaney in this category) they went too. Others I hadn't read at all; when I was a teenager I used to hit Book Attic and Book Rack and buy grocery bags of books for $15 or so. For 75 cents, you could experiment, so I picked up just about anything that looked interesting. Unless they *still* looked interesting, if I hadn't read them, they hit the sell pile. I also cleaned out a fair amount of Extruded Fantasy (or SF) Product, too. And duplicates. I don't know how I came to own three paperback copies of Song for Arbonne plus the hardback.
The mysteries got purged even more brutally. Even very good mysteries I seldom want to read more than once. All my Dick Francis I kept, of course, and Hill and Hillerman and the Yellowthread Street mysteries, but not much more. Freeling, Rendell, PD James, Martha Grimes, Ngaio Marsh, Christie, nearly every thriller... all gone. End result, of 12 boxes of sf and fantasy, five left. Of four boxes of mysteries and thrillers, three were purged. Sixteen boxes in, eight out. And I can now fit both my paperback and hardback sf and fantasy on the big set of shelves.
The really fun part was when I went to Bookbuyers. This was the crappier half of my collection, when I had duplicates I kept the nicest and dumped the more battered. Many of them had been brought used, they had not been treated gently as they travelled from storage place to storage place in slowly disintegrating boxes. I figured ten percent of them would sell, maybe a quarter if I was lucky. They took three-quarters of them. $57.20 in cash and $315 in credit. With the forty dollars I found while cleaning out the car and the change from the car and everywhere else, my Clean Sweep earnings stand at $110 cash, $315 bookbuy credit, and $300 in tax deductions.