It seems you want to create a clipboard manager. That’s not a great plan, I’m sorry to say.
You cannot listen to events happening on other applications, and there’s no “selection event” at the windowing system level. The clipboard functionality itself doesn’t work that way; you’re supposed to ask for the clipboard’s contents whenever you want to paste them.
In theory, you could use the GtkClibpboard::owner-change signal, and every time the signal fires, you retrieve the contents of the clipboard; this functionality is only ever possible on X11, though, because having random clients being able to get notifications about clipboard state from other applications isn’t really good from the perspective of a secure environment.
In practice, if you want to create a clipboard manager, you have to do so within the context of the compositor/display server.