Who guarantees you that if someone else buys the tickets? How would you go about guaranteeing it? The OP could send out pictures of the purchased ticket showing relevant/identifying inormation including a note that user XYZ has all rights to eventual winnings sans the agreed upon percentage. In combination with some escrow that holds his personal information you would have something to work with if things got hot. Then again, you will have to win the lotto first, so that's really a problem to worry about if it ever happens.. until then, precautions should be more than enough, while easy.