My question is, if I can not see who exchanged karma between each others then how would I know there was an abuse. I am sure I am missing something.
If you have some systematic -karma, if your get too much -karma and there's no good reasons for that, it can be an abuse and you can ask admin to check.
If some user gets too much karma too fast with mostly low quality posts, it also can be an abuse. admin can check.
Once caught, the abuser will get different unpleasant penalties, better not to abuse karma system. 
Sorry to say, but I personally think the karma system is too centralized, and I want to think or believe that we have more than one person behind the admin account, for if actually it's just one person, I am definitely not wishing the admin any evil, but my people have this saying that "when we prepare for good, we should also prepare for the worst just incase", so, in this regard, I wanna ask, what if anything, good or bad happens that made the admin not to come online or have access to internet for some time, maybe 1 month, 2 months, 3 months, 6 months, or even a year, does it mean the forum operations will be automatically halted?
Like for example, family traval and vacations, business trips, an accident, health issues, retirement (well, should not include retirement since this can be prepared for, and admin can hand over the forum management to another worthy candidate), but what about other possible emergencies I mentioned, and some other possible ones I forgot to?