How long are you keeping it? If it's gonna be sold in a couple years, used is fine. I would buy new if you kept it longer than that. How much are they asking? I don't like doing business with Visone as their prices are not much cheaper than new.
It is the air dryer. If air is leaking from it either the valve is dirty and won’t close or something is leaking air causing it to stay open. It is probably the latter.