If you are planning on purchasing another Tesla. Trade-in will allow you to subtract that value of your current car from the sale price of the new car. Depending on your state's sales tax on new vehicles that would likely save you $4.5-$7k in taxes. Keep that in mind when comparing trade-in versus a private sale.
Personally, I prefer to give someone else a shot at the car before trading into a dealership that is going to add another $10k to the price of the car. However, it is an option if the bids just are not there.