I suppose it's possible (and imo likely) that the used car dealership database entries can be (and again, likely are) modified by PD.
Honestly, not exactly sure what "online used car dealership means". As far as I can see, it's simply another "used car dealership", except that it differs in that
a) the selection set is targeted (sometimes it appears to be related to the seasonals, sometimes I'm not sure what the underlying motive for selections is)
b) the cars have all been factory reconditioned, as evidenced by the rollback on their odometers.
Some Standard cars are just better-looking than others. They've been serruptitiously and selectively updated a few times (beyond the 'big' interior view update), but two of any particular model are the same detail between online dealer and regular UCD and previously-purchased cars that have been lounging in your garage (other than the recent online dealer stuff being 0km and broken-in). Stuff like two-tone paints can look better than they used to two years ago (AE86 this means you), and the
Plymouth Chrysler Prowler has a real interior with working tach and speedo (formerly driven with roof up). The bad ones include the R230 Mercedeseseses, they have a blackout just under the beltline so you can't see the seats/dash/console, and those had an interior to see when the roof was down on them in GT4.
Also remember the cars in the online dealer are in your game/on your hard drive, the dealer just selects which can be purchased from there at any given time. You're not buying upgraded Standard cars just by virtue of them being in the online dealer.
I checked the online dealer & found the elise 111 R '04' , it said I didn't have it yet. Even though I was in it at the time. So I bought it & found out it was a standard version of the premium car that I was in.
The only difference was you weren't aloud to add the carbon fiber hood & full aero kit .