An Outlaws Refuge can be found in the major cities. Inside, the first quest givers can be found for the Thieves Guild and Dark Brotherhood DLCs. Fences, Moneylenders, Merchants, and Remains-Silent (with the Shadowy Supplier Dark Brotherhood skill) are also present.
Fence
Fences are the only NPCs that deal with stolen items. Sell them or have them laundered if you wish to keep the items. There is a limit to the number of items that can be sold and laundered each day, indicated under your inventory list. This amount can be upgraded with the Legerdemain Trafficker Passive Ability.