Is it possible to buy an intercity train ticket via Trenitalia? For the trip I'm looking at (Rome to Perugia) it looks like you can buy on-line for Eurostar, but the ICs say "no" under "buy."

What date/train are you looking to buy?

When I check trenitalia I see one ES with no changes at 13:46 and numerous other combinations of ES+ regional (R) or R trains only (some of which take much longer).

Only 1 IC train on the whole list for the day, and there is a shopping cart on each line for all but one of the options for the entire day.

I'm looking at October 14 at 12:14. For this route, it says "no" under "buy." But the Eurostar train at 13:46 has a shopping cart under "buy."

The 12:14 is a regional train (R), not an IC.

There are no reserved seats on the regional trains, and no advanced purchase. You have to buy a ticket in Rome. It'll take all but 2min using an automated ticket machine when you arrive at Termini, right before you get on the train.

Be sure to validate the ticket BEFORE you get on the train using one of the yellow boxes located throughout the station.

There is a hefty fine if you don't do so.

I played around a bit with dates and found that it will allow online purchase for tickets 7 days or less in advance for an R train.

Looks like you must wait til 10/07, or simply buy your ticket at the station.

There are no discounts available to the general public.

Aha. Thanks for your help.