A couple of times I’ve had this issue and while it is sometimes Forza servers, more often than not I just need to dashboard, quit the game and reboot the console.
you can always check the XBL status on their website as well.
I don’t think you’re banned. If you were banned, you wouldn’t have even been able to sell cars. Did you look under “My Auctions”? That’s where you should go to claim your credits after selling a car.
I was banned but it was lifted due an error as I’m on pc but I can sell cars but can’t view to buy then or view my auction just says server not available it worked just after ban was lifted but then an update came out and it has been like that ever since