@whybird @cstross @raymaccarthy
Not a lot, really /s
You could order Halting State and Rule 34 from the bookshop if they haven't got them in the SF/F section, but you're going to have to wait. That's for a shop selling new books.
For a second-hand charity bookshop, I don't have either of them in, and have also never seen them being donated, because people hold onto SF/F and we don't get much in, in any case. I can't order them, and even if I ask for SF/F from the warehouse, I probably won't get them. For any particular book, I can tell you if we're likely to have it, or if we definitely haven't got it, and I can check where it might be, both on the shelves and in the stockroom. I try very hard not to have two copies of any book out on the shelves and in the stockroom, because that's taking up space that could be used by another book. That's as much as I can do, because at the end of the day, it's the number of books sold that counts.
In other words, you're asking a lot!