The Guild of Thieves
The guild of thieves is much more like a coalition. Its main functions are to keep the thieves from fighting over territory and rights and to act as a middleman between the thieves and the buyers of their stolen goods. The guild is, of course, very unofficial when it comes to legal documents and being registered as such. It is more of a coalition run by a few retired or crippled thieves. Every city large enough to support more than a handful of thieves has its own private 'guild' that organizes sales and finds buyers for the goods that the thieves bring in. Of course, thieves don't have to use the guild, but thieves selling their goods without its aid often find themselves getting lower prices for the stolen items.
While each city has its own network of thieves and buyers, as well as its own small guild, the guilds of different cities often have contacts together in order to get very obtrusive items sold elsewhere. When a set of rather well-known jewels goes missing in one city, it is likely to be smugged out to another city through the guilds to be sold in another, where the price will be higher as the chance of being discovered is far lower. In this way, the guilds of each city are connected, though they each seem to run themselves separately.