This topic is brought up way too often and the conclusion is that admins/mods can't stop the black market trade of these accounts.
even if the mods could make the sale of an account extremely difficult. That would just hike up the price of the accounts and make them far more sought after.
why do you think prices would go up?
As it stands now, most sold accounts are used for non-scamming purposes, and if the sale of accounts were disallowed then the buyers who buy accounts for non-scamming purposes would no longer wish to buy accounts, but the number of accounts for sale would remain the same. Basic economics would imply that the price would go down, reducing the costs to potential scammers