You are right, the prizes aren't really useful to people that don't use Unity.
Like you said, with Unity sponsoring the prizes, and the creators the jam is built around mostly using Unity, you could argue that it is a Unity focused jam.
Maybe in the future, we could manage to get engine-neutral prizes, but that would require us to find appropriate sponsors, which, I suppose you can imagine, is not an easy task.
Other than that, the prize would be yours to do with what you please. If that means re-selling it or passing it onto the next winner, that's yours to decide upon!