Omio seems to be a legitimate site that helps you book your travel, however it is definitely worth doing some due diligence before making a booking.
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The main contenders are Omio, Rail Europe and Trainline. Those three are all legitimate companies, and established ticket resellers for European train tickets. (They're official partners of the European national train companies.) And they all put you on the same trains!
Trainline lets you choose your exact seats when booking Italian train tickets, while Omio only lets you choose a preference. (Window, middle seat, or solo seat.) They charge the same fee (roughly $2 per seat) that the train companies charge when you book seats with them directly.
“In my opinion, the best days of the week to buy train tickets are Tuesdays and Wednesdays,” said Miller. “These days tend to have lower demand for train travel compared to weekends or Mondays. Therefore, purchasing tickets on weekdays, such as Tuesday or Wednesday, can often result in better deals.