The slot is paid in full already (full price of car already paid by original buyer) so I imagine essentially it is no different than buying and gifting the car to Elon.
Not the same case with someone who didn't pay for the car yet and is trying to transfer the slot without the sales tax issues and losing the federal tax credit. I believe from another article about someone trying to sell their Model 3 reservation on Ebay, that Tesla had previously cracked down on such speculators when they found out, by canceling the order.