I know a guy who owns an armoured car. it is unarmed, but he takes it out at times and drives around.
So you can own an armoured vehicle.
One of the issues of tanks and other modern armoured vehicles is that they are *integrated systems* and the manufacturer may be able to sell you a tank, but not if it contains defense department secret technologies like range finders, sighting systems, computer driven stabilization systems, EW and comms gear, etc.
So, although perhaps you could buy such a vehicle as a raw vehicle, you couldn't buy the entire integrated array of technologies.
I think to satisfy realistic control of those technologies, you should pay (as a consumer wanting to buy one) the cost of the vehicle and the cost of extraction of those technologies from the integrated system (if even possible).
So then you could still buy an M1, but it might cost you 1.3 or 1.5x the cost of a fully-integrated standard M1.
The issue with the police being outgunned isn't on the overall scale (eventually enough ERT members will show up). It's a short (and lethal) time where patrol officers with pistols, limited armour, and unarmoured patrol cars are engaged by high velocity portable weapons systems. That's when they are outgunned and the LA bank situation was an example of that that had nothing to do with a Cartel. So would be some active shooter/terrorism examples and police are expected to be first responders here too.