I think Tesla will attempt to bring on Carmax to support this function.
On one basis or another Tesla and CarMax probably will cooperate. That already seems vestigially true with tradein valuations.
CarMax certainly could help with, say, CPO disposition, and maybe some other functions. However CarMax has only ~150 locations, all in the US and almost all in suburban major population centers. In almost all of those except TX, MI and other anti-non-franchise laws CarMax is located near existing Tesla facilities.
So, the major Tesla 2017/2018 issues in distribution and support are similar almost everywhere they operate worldwide:
1. Need about one sales facility for every ~500 vehicles sold annually, on average. Obviously they need coverage in some smaller areas, compensated by high volume major urban locations. That means roughly 1000 facilities by 2019, compared with ~200 less than six months ago.
2. Need service center or equivalent for every 1000 vehicles in service. Some other brands work with servicing for vastly higher fleet numbers but they all have vehicles, especially older ones, serviced by third parties without mfr direct control.
That suggests something in the order of 1500 facilities by end 2019.
Both of those depend largely on Model 3 buildup. The major issue is how Tesla will accomplish that and the Supercharger buildout also.
I think there will be third party cooperation on all three areas, with Superchargers almost a certainty and sales and service both certainties in several non-NA markets with fairly high probabilities of substantial cooperation with AutoNation, CarMax and/or other large multi-brand companies. Elon has dropped small hints in years past. Now something of the sort is becoming a necessity, in my opinion. As a relevant aside Mike Jackson has paid lots of attention to Tesla:
AutoNation CEO to Tesla's Elon Musk: Call me maybe!