Cars will bring what the market dictates.
If I was an individual selling a 15k miles Corolla in a private sale and know that it will bring a near new price why should I lower the price? The dealer is no different than a private seller.
A sales manager at a dealer where I once worked bought a new car (Honda) from that facility and drove it for 6 months. Someone offered him 800 dollars more than he paid for it so away it goes.
Did he rip anyone off? Not in my opinion.