The Thieves' Guild of Middle Earth was once a thriving and powerful entity. They stole to sell to black market buyers, they stole to keep for themselves, and sometimes they stole for others. They did not just steal material things such as jewels and works of art but also information. A proper thief knew not only how to infiltrate a locked door or safe but also how to infiltrate ranks and societies and systems of government. The Guild was able to create a complicated but well functioning balance. They were criminals but they were well-respected criminals with endless resources. Going after individual thieves was commonplace, going after satellite branches was ill advised, going after the heads of the Guild was insane, and going after the whole system was suicide.
When Sauron was still a man and was just beginning to plan the take over of Middle Earth one of his priorities was to enlist the Thieves' Guild as a network of spies. The payment was whatever the Guild wanted; power, wealth, a recognized kingdom of their own. The Guild sensed the evil within Sauron immediately and they refused Sauron's offer. Sauron then put a curse on them for their insolence. Every metal, every gem, every fine piece of art and cloth would turn to dust when touched by a member of the Thieves' Guild.
Being an exceptionally intelligent and resourceful people the members of the Guild did not leave time for despair but quickly summoned their brightest strategists to find a solution. It was clear that they would have to abandon their great halls, for every time they touched something it was reduced to nothing. They could not bear arms, they could not steal, they could not smith or weave themselves products. The only solution was exile until the curse was lifted.
So the Guild took to the forests. They knew their halls were safe. No one outside of the Guild knew where they were or the correct way to enter. The halls were locked up and left. The Guild fashioned new homes in the forest among the trees. To the outside world, the Thieves' Guild disappeared.