It's my understanding the maintenance plan transfers with the car upon a sale. It's associated with the car in Porsche's records by VIN number. I imagine Carmax will assign a value to it when they make you an offer just like any other option. In the end, there's probably no way to know what you're getting for the maintenance plan over and above what you would've gotten without it. I think that's true no matter who you sell the car to.