<Snipped quote by HachiRoku about Mercenaries>
That's certainly not impossible, but I don't see why anyone would do that. In a game that's not advantageous from the point of view of neither side. You can do that but probably no one will ever hire such services.
A way I see that it could work is that a client that wants a certain loot from a dungeon or mission is ill-fitted for it, would rather not risk it themselves or is simply preoccupied to do so hires this group of mercenaries. The negotiate on a first payment that could be either currency or items, and then on the rest of the loot of the dungeon, of which a large percentage goes to the mercenary group.
On the same vein, such mercenary groups could be hired for more malicious purposes, like taking out a certain player.