As an auto dealer for over 15 yrs, I may be able to help. You will always be able to get more for your trade if you sell it yourself. The real question is whether or not you want to bother with the potential buyers that will show up to your home. If they can obtain financing, are they real buyers or just kids who have no ability to buy your Z? The most accurate valuation of your car will come from either Black Book or Manheim Market Report which shows what your specific vehicle is bringing ay the auctions. Remember, tax season and year end new car specials are right around the corner and the FRS is not exactly flying off the lots right now. Hope this helps, but if you want it and can afford it, why not buy it? You probably deserve it.