Hardly "always" it's more "occasionally" that someone would be selling one.

If cycling is merely something that sounds like it could be fun, you can be patient and make the most of the used market - exactly how my parents shopped for bikes for us when we needed size upgrades, and what I did when finding one for a relative.

But someone who needs to get to work can't take such a wait and see approach if their existing bike breaks beyond repair, or more typically gets stolen.