The Black Market Merchant is your tie to the criminal underground, he will buy any weapon you have at a much higher price, like a reverse muds. Selling to him will lower your relationship with the corporations but raise your
relationship with the pirates. Selling to the BMM enough will raise your relationship with the pirates high enough that they will accept you as one of their own.
It is interesting to note that even though you may sell higher quality weapons to the merchant those weapons will never be found on pirates.