If you buy a car and run it for the whole of its life you don't need to worry about trade ins, depreciation, selling or the like.
Not a common way to own cars, the only other person I know of on this forum who's posted that they do it is
@Night Train, It means you know the exact total amount of depreciation from the time you collect the new car from the showroom. I've also given old cars to younger relatives rather than sell them at 10 to 15 years old so have had no trade in. That has allowed me consistently to get 20% off the recommended price as a cash buyer.
All that's needed is a bit of research into long term reliability, then taking care of the car when you've bought it. 150,000 miles is a reasonable life for a car (12 1/2 years ish) and on my present car that will be about £1,500 a year for depreciation.