How good are they and how do these people source it.
They are bulk purchased from either credit cards that are stolen, bogus companies being formed and asking Microsoft to provide licenses ( their technet programs) stolen from enterprises, registering NGOs, education institutes ( these two get free)
Better off using these:
MSDN accounts,
Microsoft provides keys for single time activations or unlimited activations.
Very legit but at this point just use massgrave.