My hunch (wishful thinking), is that resale values will be tied to how many of said car have been sold in the USD/LCD. Surely PD can track how many people are buying which cars. This could then factor into dynamic pricing for both the player's resale value, along with Gran Turismo's UCD.
To take this concept one step further...
Say a bunch of people are reselling 98' Integra type R's. PD could make an algorithm that takes the average miles accrued, mods made, etc.
THEN....
Next time the Integra shows up in the UCD, it might be in need of an engine and/or chassis refresh. Hell, it might come with a couple mods too.
.....Which would cause those who bought (and kept) unmolested examples of the Integra early in the game, for their cars to be worth more. Same would go IF an unmolested Integra Type R shows up in the UCD later; it'll be worth more because it is stock.