Whenever users deposited bitcoins into their account, they really sent it to an account with three keys, one given to the buyer, the seller, and the site admins.
Obviously, the site admins’ keys are compromised; however, buyers and sellers on these sites will be able to withdraw their bitcoins, or even finish their transactions, without the site admins’ keys.
Since the FBI only controls the site admins’ keys, they can’t seize the bitcoins and auction them.
Oh great. In before Lawsky makes multisig wallets illegal.