The adapter allows you to use other types and brands of L2 chargers - I'm not sure about the new fast chargers. The problem is that if you do plan to drive across the country after buying a used Tesla, you'd be limited to non-Tesla networks, most of which are a lot slower. Perhaps try asked the seller to throw in a CHAdeMO adapter for free. Yeah, I don't see why it should cost $450.
IMHO, this is the biggest reason that Tesla needs to open up its supercharger network to other cars. There needs to be an option at the charger to enter a credit card, or perhaps something could be implemented in the app to purchase supercharging with another card and VIN.
Anybody got any ideas? This is going to be a growing problem as more used Teslas hit the road.