One of the realities of a used shop is most of the time you're not going to have what someone is after. People tend to all be looking for the same handful of books at any given time, and since we can't just order more Great Gatsby, or Game of Thrones or whatever from a distributor when we run out it can be frustrating on both sides of the counter.
One thing I tell people is 'call back'. There are any number of authors & titles that we see fairly often, but also sell consistently. So it comes in, and then goes back out almost immediately. People very rarely call back. And I get why, it's effort, and the internet is right there if they're in a hurry, and if they're not in a hurry they'll probably forget about the book before long.
One such author is Chuck Palahniuk, most famous for David Fincher's adaptation of his novel Fight Club. He comes in fairly regularly, but sells almost as soon as he hits the shelves.
A while back I had a fellow come in looking for "anything by Chuck Palahniuk!"
I gave him my spiel about calling back, and to my surprise he actually did. And not just once, he called back more or less weekly for a good while..
And now he's the proud owner of Chuck Palahniuk's complete bibliography, collected a volume or two at a time over the course of a couple months.
I bet there's a lesson in there somewhere...