Say a used laptop is purchased in 2015 for 600 USD which originally was bought for 1200 USD 2014. How accurate is it to say that it's a fair price considering Moore's law, that the price should be /2 for every 18 months, would the price be even cheaper in reality because the components are actually dirt cheap?
Moore's law doesn't really apply anymore, if it ever did.
They're literally unable to make stuff twice as fast or power efficent/transistor density every 18 months.
Laptops are generally overpriced compared to what they can do, and batteries degrade as well during a year of use.
600 for a year old laptop might be a bit much imho.