mario wrote:

I'm interested in buying a NAD 521i, 521BEE or 541i
CD player. I just saw that prices there in UK are
much cheaper than here in Italy.

Do you know any good online shop that deliver to
foreign countries? I made few searches but only
audiovision explicitly says they deliver to
other countries.
Do you also know any sites with used special offers
other than ebay.co.uk?

I'm not sure whether voltage is the same as here
in Italy (220V) and if not how would I solve this
problem.

The voltage is fine.

Buy any British hifi magazine and they will have many ads.
In the meantime try:
http://www.best-hifi.co.uk/nad.html
http://uk.pricerunner.com/search?q=nad

Something to get started. Good luck.