Design contests around the UIGEA carve-out
The Unlawful Internet Gambling Enforcement Act of 2006 excludes fantasy contests from its definition of a bet when three conditions are met: prizes are set in advance and do not depend on the number of entrants or the fees paid, outcomes reflect the skill of participants and are based on the statistics of individuals across multiple real-world events, and no outcome rests on the score of a single team or a single athlete's performance. Acquirers read your contest rules against these conditions.
