  Online game involves a very large number of users who are interconnected and
interact with each other via the Internet. We studied the characteristics of
exchanging virtual goods with real money through processes called "real money
trading (RMT)." This exchange might influence online game user behaviors and
cause damage to the reputation of game companies. We examined in-game
transactions to reveal RMT by constructing a social graph of virtual goods
exchanges in an online game and identifying network communities of users.
  We analyzed approximately 6,000,000 transactions in a popular online game and
inferred RMT transactions by comparing the RMT transactions crawled from an
out-game market. Our findings are summarized as follows: (1) the size of the
RMT market could be approximately estimated; (2) professional RMT providers
typically form a specific network structure (either star-shape or chain) in the
trading network, which can be used as a clue for tracing RMT transactions; and
(3) the observed RMT market has evolved over time into a monopolized market
with a small number of large-sized virtual goods providers.
