Firstly, I must say that the comune has nothing whatsoever to do with you buying a car here in Italy. Secondly, you do not have to have "residenza" in Italy in order to buy a car here - although, as the law was only changed a couple of years ago, there may still be people that are not aware of the current legal situation.

To buy a car here, you need to have your Codice Fiscale - which is simple and easy to get. The car dealer from whom you buy your car should be aware that you can self-certify your address - a "autocertificazione". You do, of course, need an address here in order for the car to be registered - like anywhere else, in fact. If your intention is to buy a car for export to another country, you do not need the autocertificazione - but you will not have an Italian registered car then either.