You are far more likely to seat on a train next to a unicorn than to experience the possibility of having all seats sold out. It simply doesn't happen. Not even close. There are one to two trains an hour from Florence to Venice and Trenitalia (the train company) is lucky if they reach a 80% load rate. The only risk is that they run out of 2nd class tickets for the very next train. In which case you can buy a 1st class ticket (those never never sell out) or, if you don't want to spend the extra fare, buy the 2nd class ticket for a later train, 30 or 60 min later. Or if you want you can buy the tickets the day before at the station, and you are guaranteed a seat in any train.
Having said that there is an advantage in buying tickets online ahead of time. You can find discounted tickets that way (discounted tickets sell out fast). If you want discounted tickets they appear for sale 120 days prior to the date of travel. Be alert however that discounted tickets (called Economy or Super Economy) are non changeable and non refundable. If you miss that specific train you book, or if you change your mind, your money is lost.
The full price (base price) for a second class ticket is €45 pp one way. The Economy fare is €29 and the super economy fare is €19. First class tickets are about 50% more.