And when Buyer #2 discovers that Tesla only gives certain perks to the original owner who bought from them, watch the moaning here.
Remember, original buyer who immediately resells is also under a contractual obligation to not transfer their reservation. I'll leave it up to those who feel skirting contracts are a sport to have at it, at their own risk.
I think you're kind of missing the issue. Once someone buys the car they have a perfect right to re-sell it. You cannot transfer the reservation, but selling the car is not transferring the reservation.
The only real issue here is that in the case described, nobody gets the $7,500 tax credit. If the secondary buyer does not mind giving up the tax credit, in addition to paying the original buyer a mark-up, then there's no harm, no foul. I think very few people will buy the car under those circumstances, which makes it a non-issue. Assuming the seller wants $15,000 profit, and the lost tax credit is another $7,500, that means the buyer is paying $22,500 over retail price. Here's a thought experiment: Imagine Tesla announced today that anyone who wanted to pay $22,500 over retail could jump to the top of the list and get their car in a week. Would you pay it? How many people do you think would pay it? Early production cars are $49,000. But normally you'd get the tax credit, making it $41,500. Buying from a scalper would bring the price up to $64,000 because nobody gets the tax credit. Effectively you're paying $22,500 more than if you bought the car from Tesla. You might as well buy a Model S, get a bigger, faster, fancier car. Nobody is going to take this deal.
If one or the other of them (or both!) try to claim the tax credit, then they're messing with the IRS. They might get away with it, or they might not. I'd say the consequences of getting caught are not worth the risk.
If someone made a reservation to buy the car as a gift for their son or daughter, I don't think Tesla would regard that as a violation of their intent to prevent scalping. Nor would, I think, any reasonable person. I have no idea how the IRS would treat that case.