Will you buy the old, outdated used books?

Old, yes. Outdated, depends on the subject.

Computers - I read old books only if they're still relevant. For instance, my copy of the K&R C book (one of the first printings of the second edition) dates back to 1986 or 1987, while they were finalizing the ANSI C standard. Every line in the book is still relevant, however, so I use it. And I don't own a FORTRAN book newer than 25 years old. But on other topics, like hardware and software guides, I avoid the obsolete stuff.

Fiction - I read anything and everything. A lot of the best Science Fiction came out of the 60s and 70s. A lot of it relies on scientific concepts that were later proved inaccurate, but it's still fun reading. (Just like I still enjoy watching Star Trek TOS, even though their understanding of things like Warp Speeds and Antimatter is critically flawed in some episodes).

Non-Fiction, Political Science - Sometimes I read these just to get an understanding of how people used to think and to reflect on how we got to where we are today.

Jam it back in, in the dark.