Have you not looked at the several threads involving sMerit farming? You do realize these farmers don't just have 3-4 accounts, they have many more. Once those sMerits are used to level up a few chosen sig campaign accounts, off they go to buying more accounts with sMerits. So it would be safe to say the behind the scenes of account sales are surging and there is no short supply of account sellers.
If it has not already happened, the Merit source accounts will be bought and sold so someone with a little savvy will trickle sell the sMerits. Catching these will depend on just how savvy they are.
I always thought most of the accounts that are sold by account sellers are those hacked accounts. It might be possible that when that hack happened, some people noted the account details of high rank accounts and then changed the passwords. I am not sure of this but why will anyone sell an account which he has worked on. Does not it makes sense to earn with that account from bounties. And now that we need merits for leveling up, this account farming will come to an end finally (especially for ranks like Sr member and above).