Quoting the ToO Raffle rules:
The value of your bought tickets will go the Jackpot. The Jackpot already holds 10.00 $ per round. The more players the round has, the higher the Jackpot gets of course. After the draw period a winner (or even none) is selected by random. If no one wins, the Jackpot will grow even more. So good luck!

Regarding that bold bit: Under what circumstances would no winner be selected, and what's the exact likelihood of that outcome (if an exact likelihood can be determined)?