If the person used all the mounts first, or isn't waiting for the timer that clearly says bound for XXX, then it's a ID 10 T error.
There is no way to use a mount and still have it in your inventory, the item disappears.
OP, you may be bugged, I would put in a ticket, as far as not being able to list vehicle on the GTN. However, that does not excuse you from ranting about not being able to sell Revan's Mask, when it clearly is an easy sell for even 200k. I have never had an issue selling anything from the packs via trade chat. And it doesn't take very long either.