It wasn't really random in GT5. It was just a list that it slowly cycled through. I forget how many cars were in there at once and exactly how many came and went at once, but I think it had thirty-six at once, with the six at the bottom being shoved out and six added at the top every time you left an event. The order was the same every time.
Two important facts need to be pointed out here:
1) The used car dealership was simply a way of making the 800+ sub-Standard last-gen carry-overs available in the game.
2) It was not meant to adequately simulate an actual used car dealership, wherewith you have no say on what shows up and anything can turn up at any time. Arguing that it shouldn't make all Standards available at once because that is unrealistic is a pointless argument since it doesn't act like a real UCD anyway. Further, some cars in the UCD had zero miles, as I recall.
Just make all the last-gen stuff available all at once and be done with it.