We'll be traveling in Italy in September and need to get from Venice to Rome for the second leg of the trip. Should I buy the train ticket in advance or is that something I can easily purchase once in Venice? Thanks!
Purchase the ticket in Venice, no question about it. The only time you might want it booked in advance is if it is a large group travelling and you need seat reservations for all of you to be together.
The trains that run from Venice to Rome are the Eurostar Italia (ES) trains and they require a seat reservation, which is included in the cost of your ticket and is reserved at the time of purchase.
When buying your tickets, you can also do it from an automated machine... and if you want to you can purchase it at the machine (or a ticket window) in Venice in the days before you depart... just remember that you need to reserve for a specific date and time.
For details on how to use the automated machines, take a look at this excellent walkthrough.
Have a great trip!
In September you shouldn't have trouble getting a seat on the train without buying your ticket in advance. To be safe why not buy it the day before or the morning of travel. Most travelers suggest 2nd class. I travel 2nd class without a problem.