see here: https://www.reddit.com/r/DarkNetMarkets/comments/76kvnh/got_a_doxx_letter_in_the_mail/
People in the DNM thread are asking him what cali vendors he's used/ or that a vendor in the past is responsible.
I wouldn't be surprised if these letters are a (free)price you pay for using a market's built in encryption. You may not be at risk by the sender, but you are definitely a risk to yourself. If your opsec is so piss-poor that you're getting physical letters in the mail accusing you of DN dealings YOU SHOULD ABSOLUTELY BE CONCERNED AND IMMEDIATELY CHANGE YOUR HABITS.
you are probably being extorted by incompetent Dutch LE
Stay safe kiddos
That or vendor sells customer info and it's still not the vendor blackmailing you. Then it can come from any state.