Isn't looking at Craigslist selection bias around people looking to buy/sell cheaper cars without warranties?
If I had a used car worth 20K still in a maintenance window I wouldn't sell it on Craigslist. Autotrader if I wanted to private-party to max my upside, dealer if I was lazy.
Very much selection bias. Even if we add auto shopper, and the like you won't get most of the expensive cars as they are not advertised.
Your 20k used car is 2-3 years old and coming off lease, or the same car as a trade in. The dealer will put it on the lot with a "just off trade" sticker and sell it. It might or might not go on Craigslist. (if it is listed the ad finishes with "and many more cars like this so some in and see") If the dealer gets too many in on trade the rest go to an auto auction.
Remember, the dealer knows all the numbers that the leaser isn't told. The company who backed the lease 2-3 years ago did their best to make sure that when the car was turned in the dealer would look at the remaining value and think "profit if I buy that and sell it". The dealer is going to pay the lease company for the rest of the value of the car. If the dealer decides there isn't enough money in selling it the lease company has to send the car to an auto auction and see what price they can get. (I suspect that a lot more goes on in these cases but I don't know what)
If I had a used car worth 20K still in a maintenance window I wouldn't sell it on Craigslist. Autotrader if I wanted to private-party to max my upside, dealer if I was lazy.