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Tesla Switches Full Self-Driving to Subscription Only
(consumerrights.wiki)
This is a most excellent place for technology news and articles.
Here ya go: https://www.jalopnik.com/tesla-remotely-removes-autopilot-features-from-customer-1841472617/
Edit: Ah, sold privately, as in directly between individuals? I'm not sure about that, but they totally could. They can track the location of the car; if it starts getting parked at a different house regularly, for example, it'd be easy to tell it'd been sold.
Yeah they probably could, like all manufactures can these days, but I haven't heard of it actually happening outside of them going back in to Tesla to be resold. Directly from person to person they retain features and even free supercharge access.