I really like this idea.
But the issue with that is Mihawk would also be a part of this underground trade. And I personally can't see Mihawk taking part in this type of an organization, especially if he doesn't benefit from it somehow.
Underground swordhunter, it'd be the easiest way to find those special swords(can't remember the name but the ones Zoro has 2 of and there's like 33 or 36 of them or something) He could also just use it as an information network.
My reasoning behind it being underground market related is this: Blueno and Mihawk have it on their left arm while Doffy has his on his right. Doffy is Joker a big time seller in the underground, maybe the right arm is a symbol of power and the left arm means a member of the community or something.