  Propagating information to more people through their friends is becoming an
increasingly important technology used in domains such as blockchain,
advertising, and social media. To incentivize people to broadcast the
information, the designer may use a monetary rewarding scheme, which specifies
who gets how much, to compensate for the propagation. Several properties are
desirable for the rewarding scheme, such as budget feasible, individually
rational, incentive compatible, and Sybil-proof. In this work, we design a free
market with lotteries, where every participant can decide by herself how much
of the reward she wants to withhold before propagating to others. We show that
in the free market, the participants have a strong incentive to maximally
propagate the information and all the above properties are satisfied
automatically.
