Under European law tax is payable in the country of registration if it is a new means of transport. Even if tax has been paid in another country. In theory you should be able to get the tax refunded if it has been paid. If it has never been registered this is not a problem as the tax either has not yet been paid or the seller can claim a credit. If it is a near new second hand vehicle it is still considered new and I can not imagine the difficulties one would have trying to get a refund out of the Italian government.
From
http://customs.hmrc.gov.uk/channelsPorta...000165#P35_32222.2 When is a means of transport not new?
A ship or aircraft or a motorised land vehicle is not new when all the following conditions are met:
when more than 6 months have elapsed since the date of its first entry into service (see paragraph 2.3), and
it has, since its first entry into service, travelled under its own power for more than 6000 kilometres.
So one would need to make sure both conditions are met. In the case of the Plus 4 SS in Milan it would need another 2,000 km on the clock.