My main contribution is: never trade in your existing vehicle if you can avoid it. You will always always always get fucked on the deal.
I walked on a basically done deal a few years ago over this. The new car sales manager offered me $4500 for my trade, which I snap accepted because the truck was 10 years old, had a zillion miles on it and the AC was busted (on Dodge pickups, in order to replace the compressor you essentially have to disassemble the entire dashboard and get to it from inside the cab of the vehicle…VERY labor intensive and it was going to cost a ton to get it fixed). By the time ol’ F&I got his meat hooks in me, the $4500 had been completely forgotten by everyone but me. They couldn’t honor the manager’s “offer” so I bolted. Though I had to wait like 20 minutes for my keys…they like to take your keys away to make it harder to change your mind or something.
Now sometimes you will almost have to trade. Like when your only vehicle is the one you’re considering trading, and selling to a third party means renting a car until you can buy another one or something. In that case, settle on a price first and don’t even mention a trade. Once you’ve dickered down to a price you’re comfy with THEN and only then ask them what they will give you for your trade. That keeps their fuzzy math out of the deal somewhat.
But mainly just sell your car to a private party and don’t trade it because more often than not you’re literally just giving them your existing car for free if you don’t know what you’re doing.