Hi i am looking at this 350z for sell and wondered what is this copper tubing in the engine bay as i have never seen it before.

 

Grounding kit

 

Yep, home brew grounding kit using what appears to be stereo/amplifier heavy gauge power cables. No big deal, leave it be or remove it. If you live in a rust belt area these modification are a good insurance policy. If you live in a dry climate like Nevada or Arizona, this isn't really necessary as the factory ground wiring is adequate. Usually cleaning up the main battery terminals and applying some dielectric grease is all that's needed to keep the pixies flowing.
Cheers!
-Icer