So, if you write for a small local paper, you're not a journalist?

More like if it's an internal paper that only selected employees are allowed to read. A small local paper can still be read by anybody in the world.

But if a spy sells secrets to anyone who'll pay for them, is he/she a journalist? :)

If it is broadly known that she's willing to sell those secrets to anyone, AND if she allows the stories to become widespread i.e. everyone can buy the story, not just the highest bidder, then yes, it seems to me that she's essentially operating a (probably) very expensive bulletin.