It could be many things peachy, it could just need a service(cheap), it could be a blown cartrige(expensive), It could just mean the adjuster is siezed..............
What model Z1 is it ? I've seen people try to sell a Z1 drop off(aka, a Z3) as a propper Z1, don't let em fool ya.
personally i'd steer clear, if it was an easy fix wouldn't he have fixed it for a faster sale ?! Maybe ask him to have it fixed first and see what he says.