If you priced carbon emissions for building the car into the cost of new cars, it would fix this. In the case of your example, the used car would be worth more

If you priced carbon emissions for building the car into the cost of new cars, it would fix this. In the case of your example, the used car would be worth more than $8k because new cars would be correspondingly more expensive. Of course the problem is as you said how do you price carbon emissions.