I know this is a old post but I just want to share my experience on tickets purchase on Trenitalia website. Like many others, I have tried all of my non-italia credit cards and none of them worked. I even called Fraud department but could not find any payment denied attempts.
I called the trenitalia customer service several times ... I got connected only once and after 20 mins of discussion (taking my name, trip details,etc I had to repeat my answers many times though, understood that english not their native language), the representative told me that there are no tickets available and just hung up. I checked online, but still able to see seats available.
I even requested my hotel to book tickets for me, but they politely said they cannot book for others.
I changed browsers, cleared the cache, tried to book from work, reserved via postoclick and tried to buy next day, NONE worked.
FINALLY, I wanted to try the virtual account number before buying tickets as soon as I land in Italy. Hurray! it did WORKED this time. I was able to book train tickets online by using a virtual credit card number (from citi) and saved 23 Euros !!!
I hope they will fix this one day, at least in the near future.
buona fortuna !!!