So six of the seven preorder cars were added to the Horizon Backstage (the Porsche still has to be unlocked in The Eliminator).
What’s the reason three of these (Aston Martin, Chevrolet and McLaren) can be bought in the auction house, while the other four (Ford, Mercedes-AMG, Nissan and Porsche) are still blacklisted there?
Intersting 
. I know the reason for the Backstage Pass was to allow users to not lose their chances on obtaining rare cars again. As the playlist is over, the cars were really rare and seasonal playlist exclusive and will not reappear there. Adding them to the Backstage pass saves time from trying to stress/grind for them via the auction house.
Can you reverify that the Ford is blacklisted? I understand that the Mercedes AMG, Nissan, and Porsche are because they are readily obtainable. Porsche via Rank 20 in the Eliminator, and the Mercedes AMG and Nissan were gifted to everyone via the message center (they were a reward in Sept of 2019 during the Forza Monthly for everyone who watched, but there was an issue and everyone got them). I am not sure why the Ford would be blacklisted.
Just tried again and still getting a “server communication error” when trying to auction off a Ford, Porsche, Nissan or Mercedes-AMG.