  Rating systems play a vital role in the exponential growth of
service-oriented markets. As highly rated online services usually receive
substantial revenue in the markets, malicious sellers seek to boost their
service evaluation by manipulating the rating system with fake ratings. One
effective way to improve the service evaluation is to hire fake rating
providers by bribery. The fake ratings given by the bribed buyers influence the
evaluation of the service, which further impacts the decision-making of
potential buyers. In this paper, we study the bribery of a rating system with
multiple sellers and buyers via a game-theoretic perspective. In detail, we
examine whether there exists an equilibrium state in the market in which the
rating system is expected to be bribery-proof: no bribery strategy yields a
strictly positive gain. We first collect real-world data for modeling the
bribery problem in rating systems. On top of that, we analyze the problem of
bribery in a rating system as a static game. From our analysis, we conclude
that at least a Nash equilibrium can be reached in the bribery game of rating
systems.
