Buy train tickets in USA or ??? I'm nervous

Can I buy point-to-point tickets there?

Keep in mind that RailEurope is a travel agency and charges high fees for their services. And you can buy nearly any ticket there.

I used rail europe and they were very easy and not very expensive. I wanted to have train tickets on a specific time and it was ease of mind to know I already had tickets before I left so I got the specific time I wanted.

just go to raileurope.com

For the most part, I'd go with the ETBD's Rail Dept recommendations. I, too, am a compulsive planner, and I like to have everything nailed down before I go over there. However, I have never had a problem waiting until I get there to buy tickets.

If you are worried about tickets in Italy, buy them there the day before. You will get a 20% Amica Fare discount. In almost every case that will assure you of a seat.

As for Venice to Munich on a night train, that is a very popular route, and reservations tend to sell out. Make those in advance from Bahn.de. I don't think RailEurope even sells them.

Good advice from the RS site also is, if there is a train you MUST take (like your Venice one) at a certain time, it is worth doing ahead, especially if you can get a reservation.

I used RailEurope to get our tickets ahead of time, for Italy. It worked well for the situation like yours (ours was Venice to Rome), but I ended up being sorry I had done the "point-to-point" ahead of time. We lost some flexibility, and, most local trains don't let you make reservations anyway. It really is easy to get them at the stations. One time a train I had advance tickets for was cancelled part- way through, and it took forever back at home to get just a partial refund from RailEurope.

Another option is to get your shorter distance train tickets for your next "leg" when you arrive at the station. You'll have the ability to look at all the schedules and still have them ahead of time. I did that a lot on my last trip to France, especially if the station wasn't that close to the hotel

You should have the Venice to Munich night train tickets in hand before you travel. Night trains can sell out. On any other day trains in Italy, just buy your point-to-point tickets a day or two ahead of time. Unless it's a major holiday, the trains do not fill up. The route from Rome to Florence is one of the most traveled train routes in Europe. 12-15 high-speed trains do this journey each day. It's a 95-minute trip. It's like Fresno to Bakersfield.

Travelers freak out about "reservations". Don't worry. It's not like plane reservations. It's like getting on your local bus system except that they assign you a seat. When you buy your ticket, you get the seat reservation automatically from either the ticket agent or the self-service machine. On the slower local trains, no reservations are permitted. You just validate your ticket and get on. If the seats are full (which they won't be), you stand.

Whether you buy here or there depends on when you travel, the time needed trains will depart and your need or desire for special accomodations.
As has been said, if you MUST be on a given train, it is wide to buy way ahead to ensure your choice. If you have time btw arrival in Europe and taking that train, you will likely get what you want even by buying there, unless it is a holiday train or popular route.
You say you want a "comfy seat" on an overnight train.
This should not be a problem. What generally sells out are the sleeping places

Paul, Karen, Larry are all making good points. And all experienced train travelers will agree with them. Since we are in education and forced to travel in the summer, we have never encountered a full train. Crowded on holidays - yes - but never complete sold out but I am sure it can happen. We traveled last summer. end of June Rome to Florence, Florence to Milan, and Milan to Zug (Zurich) -- all second class -- and at no time was our car more than half full.

I can understand your apprehensiveness until you have actually had the experience. Unfortunately, as Americans, we trained by our airlines to make reservations early because the capacity is not there to move a lot of last minute people. If the European railroads followed the Am airline example, they would offer one small train to Florence each morning and maybe one in the afternoon. Don't remember the exact schedule, but I think it is a train an hour to Florence, maybe more.