That would be included in the for sale price. For example, the trade in value for the 350z might be $10K, but the private party value of the vehicle could be $12,750. If you owe $6,000 on the vehicle and sell it for $12k, you would pay $6K to the loan company, and you could use the remaining $6K as a down payment on the 370z. However, using the trade in method you would have less to apply toward the 370z.