Car trading limited by Patch 1.07
The 47 MB patch is now available for download when you launch the game.
You can read the full patch notes here. Among changes on the list appears a limit to trading of cars above 1 000 000 Cr value.
The value of a Used car is not based on the price you buy it, but on an invisible internal value which is slightly higher ; concesquently, most cars that are priced around 980 000 - 999 999 Cr can't be trade either. Here's a link to the GT5 car list ordered from most to less expensive.
UPDATE : The "my page" and compare garage page now filters cars you can't trade anymore.