How do I determine sales price for traded car?

The trade-in value that you received will be the sale price for your traded car. 

@Leonard what if I traded in my car and used the amount/sale price(14000) to purchase another car and also gave 2000 as a down payment needed to get the new vehicle. Would my expense be $14,000 + $2,000?

No. The value of the old car is still $14,000. That's the amount the dealer is giving you credit for.