The Thieves Guild 
When the merchants become rich, so do the thieves. Indeed, some of the merchants may even be thieves themselves. Secondary in wealth to the Merchants Guild, but certainly more deadly and dubious, the Thieves Guild oversees the underground black market of Marikests back-alleys and hidden streets. Every major town has its own black marketeers, fences and worse. In some smaller towns the Thieves Guild is actually the center of power and the Mayor or town council is either puppets or full-fledged members. 
The larger towns and cities have more complex black markets, with everything ranging from prostitution to trade in the dark arts. Not all of the people in a local thieves guild is evil, but they do turn a blind eye to the corruption they see around them. 
For example, in coastal towns like Weyvin and Koastmark, where prostitution is more common because of the sailors the local law enforcement turns a blind eye, as do the dock officials and many others. In towns like Oraknev and Dirvik however, prostitution is firmly outlawed and the pimps and brothel matrons who run the show are dealt with swiftly. 
Everything is always cheaper on the black market (anywhere from 20% to even 70% off, depending on how desperate the seller is to get rid of a hot item). You name an item and even if it’s not in stock, you can make an order for it to be found (with payment on delivery, of course). Contract theft is a growing crime between rival merchant caravans. 
Bounty hunters are often members of the Thieves Guild, but maintain a low but impressive profile as their occupation often means they must go hunting for a fellow thief. 
Pirates often associate themselves with the Thieves Guild, but usually only to sell off their stolen goods. 
Mercenaries often keep contacts within local thief groups, in hopes of gaining extra employment. 
The secretive nature of the Thieves Guild drives them to set-up legitimate businesses as fronts and then sells their stolen goods under the table. They often have a complex network of thugs, front men and bribed john-laws that allow them to operate. Organized gangs of thugs and/or burglars are becoming more common and efficient, but also sometimes have gang wars. Behind the scenes, crime lords with their own internal networks have begun fighting over who has the right to be on top. It’s not a fight of good against evil; it is dog eat dog.