Ry Hatch
There is so much misinformation in the answers here. As a former technical production manager at carmax, I'll clear it up. This is completely unbiased and from one consumer to another. I left carmax on bad terms, but would still highly recommend the extended service plan (maxcare) from them.
1. CNA is the company who covers the vehicle sold by carmax. It can be used at BMW, carmax, or any repair facility that offers a 6 month and 6k mile warranty on their repairs parts and labor.
2. Carmax doesn't only cover/repair ICE vehicles; they repair Ev's, hybrids, and of course, the i3. Their technicians are ASE certified and the vehicle will go to a tech who can repair it. In the event their facility can't repair it, carmax will have the vehicle taken to a facility that can (i.e. BMW).
3. Maxcare is a named exclusion policy. This means that they specifically name the components that ARE EXCLUDED. All other components are included. A few components that aren't included: battery, shocks (struts/coilovers are included), battery, brake pads/rotors, body panels. Etc. These are wear items or components that you don't have.
4. You SHOULD get the vehicle from carmax and you SHOULD get the extended service plan. Many BMW, Mercedes, Audi, jaguar, and land rover techs and employees would purchase their vehicles from carmax just for the extended service plan.
5. A CPO from BMW only means that the vehicle met mileage and age requirements, nothing more. BMW includes a limited warranty and charges $3k-$7k more for the CPO name.
If you have any questions, feel free to ask. Again, I'm no longer with carmax and didn't leave on good terms, but they're the best when it comes to vehicles and the extended service plan.