If you can't see the used one filled and running, it's worth zero. Even filled and running, it's worth less than half of what a new one is considering there is no warranty and you will have to pay to move it.
2018 has dual filter vs single filter, exterior lights, different exterior (easier to remove and looks better in my opinion), Ozone standard vs optional, warranty vs no warranty, updated styling which in my opinion provides a little more interior room.