The Merchants’ Guild hire their own memebers for bounty missions. Why? Because the Merchants’ Guild is too big to be dealing with one case of piracy at a time. They need a blanket solution, and so what they do is pay a monthly fee to the mobs and/or pirates in exchange for blanket safety to their ships.
That, plus the fact that they do shipping for the mobs as well. Except Lynch, that is, who owns his own shipping company: Lynch Shipping. (Or, well, at least we know they work with Lang; whom Lynch considers his nemesis.) Not that the Merchants’ Guild don’t get attacked by independent pirates, but that they can manage to control that with the help of their own bounty-relishing members.
And what they can’t handle through their members, they have Lynn Murphy for. Probably ex-militia, Lynn is likely the chief of Trading Lanes Safety for the Merchants’ Guild. It’s probably why hunters won’t work for her; they simply don’t do business with the Merchant’s Guild.
Do not confuse the MG with “merchants” in general. There are many independent merchants out there. Lynch Shipping comes to mind...