I am going to be in Italy the first two weeks of November, Rome to Florence on a Monday morning Florence to Venice on Thursday morning
Venice back to Rome on Friday afternoon. Should i book tickets now from the US or just buy tickets at the station when I am ready to travel.
These routes have many trains each day, so you'll have no trouble getting on one. The issue is cost. If you can commit to a specific train, and to non-exchangeable, non-refundable tickets, you can save money. If you don't mind the full price, you can just wait until you get to Italy to buy tickets (you can buy tickets for any route at any station). Here is Ron in Rome's take on the latest Italian train prices.
As Harold suggests, it is all down to your comfort level. Do you prefer to save money, locking in date and time - nonrefundable - and able to print the tickets at home (slightly fiddly and sometimes takes a few tries), or would you prefer to take the train you can decide on the moment but dealing with queues at the station and possibly credit card issues at the machines, and paying much more by paying full price. It is entirely your choice. Both ways work.
If time is no object, you can take regional fast trains last minute for less money. They're 3 1/2 hour Rome to Florence vs 1 1/2 hour intercity express. 19 and change vs 29 advance economy.