I tried going to the trenitalia site to buy tickets to go from Rome to Naples but it wouldn't let me buy them. It said I had to buy them at a station or through an authorized Trenitalia travel agency??? Can someone out there help me???

An easy way to buy in Italy is upon landing at Rome's airport - at the airport train station - as there are two trains an hour Rome-Naples you shouldn't have a problem reserving the train you want - you could possible save a few euro with PREM fares on treinitalia site but for short trips why hassle with it?
I'm sure someone will point out a glitch so you will be able to buy them on the site.

You can't book more than 60 days in advance. When are you traveling?

We bought tickets from Venice to Bologna and back and Venice to Verona and back right from a self serve kiosk in the train station and it was really easy-hit the English icon and the whole thing was in English. They also had several ticket agents as well. Unless you are nervous about your travel dates, i.e. travelling during an holiday, etc. I'd just buy them there.

It's very easy to purchase tickets from the automated machines at the train station. You shouldn't have any problems getting a seat on the train you wish because there are many that follow that route. (like PalQ said)
All of the instructions on the machine can be in English, so as long as you know the route and time you want to travel, you should have an easy go. You can usually pick your seat out with a handy little illustration of the inside of the train car as well.
Just don't forget to punch your ticket in the time stamps at the station before you board the train!

Good call TravGrl-we got "busted" on our first trip b/c we forgot to stamp our tickets.

I've been looking at the trenitalia site to purchase tickets. We are travelling Rome - Venice - Florence - Rome (our travel plans are another story).
On the website it has "venice link: Eurostar + motor - boat in Venice".
I don't understand the explanation they give, but it sounds like it includes the Eurostar train from Rome to Venice and then the boat from the train station to your hotel. Is this correct?
And, by reading the latest posts, is it better to buy on trenitalia or at the station. We'd like to travel 1st class Eurostar.
Thanks.