I still think you're missing the point everyone's trying to make about how the Used Car dealership works in this and previous versions. It only displays a set number of cars and those cars change as game time passes, it's always been that way and GT5 is no different.
Perhaps what you're seeing is a difference in the stock the main dealerships have, back in the previous games the Manufacturer specific dealerships often had the current / newer models available which were different then than they are now (e.g. GT4 had a '05 Ford Mustang as a "new" car to be brought new from the dealership (in GT2 the '98 Mustang was the current one, I think), in GT5 it's considered a Used Car though I think you can buy the '08 model from the dealership.
All the cars that aren't considered "new" and sold through the manufacturer dealership are sold as "second hand" cars through the Used Car lot. So you want to buy lets say a '98 Subaru Imprezza STi (or a Mitsubish Lancer Evo V as I did), well that's not a new car any more so you'll have to buy it second hand which means the Used Car lot but there's a good chance that it won't be available there right away so you have to do some races (or buy something else if it's your first car) and wait for the Used Car lot to refresh it's inventory with a new set of cars for sale. All the GT games with Used Car lots worked like this so you've never been able to buy just any car any time you wanted, nothing's changed there.
"Not visualising the equine bestowment via it's verbilisation aperture!" "Every silver lining has a touch of grey.. "