This approach, listing up all transactions of large amount of merits, should be regarded as the first step, as it omits cases such as multiple transactions with e.g. 20 merits between the same users.  
It is also helpful to check users who send merits back and forth among particular closed users, which are most likely merit farmers.  
To detect such abuses, we need more careful investigations by taking into account not only sent merit amount but also user data.