What I think will be interesting, is what sort of cars will be in the used dealership? In GT4, it featured many variants of certain vehicles, sometimes with minimal differences beyond market (e.g. US/EU), and sometimes even having less-desirable variants of cars. Meanwhile, in GT5, it was basically a dumping ground for the standard models, and I don't recall it being in GT6. But that said, I think at the same time, a lot of interesting older models were exclusive to the used dealership in GT4, so I think this trend should continue in GT7.
I believe like all previous iterations, the used car dealership in GT7 will rotate its stock from time to time. Given that it's highly improbable that GT7 will feature anything even resembling a standard car, I bet that there won't be so many potential used cars insofar that the used car dealership will need to be split up into sub-groups, as it was in GT4. I also believe a more limited number of cars will be available at the same time - I'd argue you could only see thirty at a time in GT5 because of how the UCD was a dumping ground for standard models, of which there were literal hundreds. And also like previous iterations, I believe the cars found in the UCD will be exclusive to it, and you won't be able to find them at the dealership.
I also would not mind it at all if certain cars with special liveries were exclusive to the UCD; in lieu of the four black cars from GT4, perhaps this could be where you find the cars with the "chrome line" and "stealth" liveries from GT5, alongside the cars from GT6 that donned the 15th anniversary livery.
At any rate, I definitely look forward to likely buying my first car in GT7 within the UCD, even if we're given one from the get-go like in GT6 and GT Sport. If that's the case, then depending on what models are available with the starting credits, and which model might be given to us to start, I might leave one stock and modify the other, and essentially refurbish the used car I buy either way, at least changing its oil and restoring rigidity. (That is, if those are factors in GT7's used cars.)
I could definitely see a combination of common and rare cars being available via the UCD, but I really do wonder how large the selection will be in both the UCD and the dealerships. I can only assume there are enough cars in the game that it warrants this "split" between the dealerships and the UCD. That could bode well for those wishing for a notably larger car list.