There is really no way to eliminate this type of behavior. One poster commented about stopping currency transfers between players. People would then begin trading in items. Much like the "stone of jordan" in Diablo 2. It has happened in other games as well.
The most they can do is too monitor large sums of money or several smaller sums of money over small time frames. It is done in the real world. I believe the US Federal Governemt tracks any money transfer over 10,000.
Another poster mentioned using a reward system to turn other players in. That is not something you want to begin. That only turns your player base against each other. Using the same greed that promotes gold selling.