I shall take great care to safeguard my account.
Do you monitor for suspicious diamond purchases that do not correlate correctly with purchase data and reward data. Can you cross reference, While catching hackers is hard, analyzing and auditing your own raw data should not be that hard, it can find patterns and trends and reveal unusual activities.
You would also want to monitor purchases at the black market that do not match, the audit data you have.
Then finally after all those hurdles are passed and most importantly monitor each players army size and content to see if it is above average in growth minus the previous audits.
Once you have a lead then you can find the trail and then catch the money train.
I was tempted to study cyber security when I was younger, in retrospect i looked too far ahead and saw programming as the cornerstone of fraud detection and too great a challenge. However it may actually be more a of a process of forensic auditing and data comparison rather than hardcore programming. I only got as far as pascal and C++ so i found programming too daunting...but my strongest skill was in statistics, which it dawns on me is the very core of cyber security.