There are rumours that Greek banks have also been keen sellers of credit-default swaps on sovereign Greek debt, in effect doubling up on their exposure to a debt crisis.

ECONOMIST: Sovereign risk and the banks

Credit default swaps are generally traded by phone-based brokers who match buyers and sellers at large investment banks and other institutions.

FORBES: Magazine Article

Hit by a series of scandals and highly-publicized attacks by opportunistic short-sellers like Citron Research and Muddy Waters, investor sentiment has weakened and doubt has become the default position.

FORBES: Short-Seller Attacks Highlight Chinese Companies Social Media Shortcomings