I know the e9x m3 prices were dropping hard but thats ridiculous. Apart from the mods seems ok. Give him a call and take it for a spin. Under warranty with bm too so give them a call. Still seems cheap to me.
Check if he is running budget tires. Usually a sign that its prob also tun on 95 Ron fuel. If this wad the case is prob walk away. In fact I wud walk away and wait for the gtr.