Used cars? Insurances?
Of course the used car market won't be touched. There are not much used cars from the EU with the driving seat on the wrong side <G> That's mostly a fully internal market.
About insurances, coverage outside the UK will need to be discussed. Right now an EU insurance covers you in any EU country - no need of the "green card" or whatever is called in English.
I believe there will be an agreement to ensure it, but it also mean that there will still be rules to abide - be them EU-UK agreements or the "green card" system, which may still be an obstacle for smaller insurance companies.
Medical assistance abroad for citizens of both "areas" will need to be discussed, and if not free, travelers in both directions will need an additional insurance.