The best way to go about it imo is to gain the respect of seller, whether its a private sale or a dealer.
Phone up before viewing the car, get a handle on the details of the car and arrange a time to view. Don't talk money over the phone.
Turn up on time. View and test drive the car. Assuming you like the car, only then start talking money.
There is nothing worse than the knob heads who text or email offers of substantially lower than the asking price, without even seeing the car!!!!!! I refuse to engage anyone in bids for the car unless they have seen it or prepared to offer the asking price.
What amazed me was when I was selling the Gallardo that people would actually text me offering £80k for the car, without even seeing it or asking any details of the car!?!? WHo buys an £80k+ car in that way?!?!?!?
As RT says, you need justification for your haggling, be it the condition of the car or cheaper cars elsewhere.
Also don't assume that a dealer has £5k of profit in a 350z, some dealers knock out cars with less than £1k of profit, which after paying overheads doesn't amount to a great deal......