Here is what I believe:
Seller "A" has a farm of accounts, and he makes mistakes so his accounts are connected.
Account seller/farmer "B" has a farm of accounts, and he makes mistakes so all his accounts are connected.
Buyer/spammer "C" buys one account from both "A" and "B" and he makes mistakes so both of his accounts are connected.
It now appears as if one person owns all of "A"'s and "B"'s accounts, but this is not actually true.

In order to confirm if two accounts are controlled by the same person, you need to rule out that any of the accounts had changed hands after the date of the connection.