I've had a lot more luck with science fiction in professional shops than in charity shops. In the latter I tend to find only mainstream fiction and non-fiction. It's exceedingly rare to find any sci-fi at all. I wish I knew one where people brought it in.

One thing I enjoy is when people get rid of their computer science textbooks. They are sometimes older editions, but can be had for less than 10% of the original price. It's rare, though, so just have to keep going and take the opportunity when it arises.