I've noticed something about this dealer. You can only buy one of each car from the dealer, and mentions upon doing so that you have reached the limit of this type of car you can own.
This may mean that players can only have (or at least buy) ONE of each Exclusive Car. This could be Turn 10's way to circumvent the problem with FH3, where some players would buy multiple copies of Forzathon or otherwise limited cars and then resell them for a massive profit on the Auction House.
The AH might be a possible exception because I don't know (no one does at the moment) if you can buy, for example, a Showcase car before you've actually done that car's Showcase Event. If you can, that would allow you to have duplicate Exclusive Cars. Winning one from a Prize Crate may also be an exception, but of the three I've bought, I haven't won an Exclusive Car in any of them.
One of "The 50"