-Or the assassin market. How many people wanted to hire assassins but were assassinated upon payment before crypto-escrow? I bet more than one.
Someone needs to start a legitimate darkmarket for this. I'd have the OP killed...but probably only receive "proof" photos from an FBI agent.
That reminds me of the guy who put his DNA sequence on some kind of blockchain... In the future, I'd guess assassins could send you DNA proof in the form of something much smaller than a severed head which you could compare against their public DNA sequence. ..... .... You know, with that DNA sequencer you had made up at the local bitcoin-accepting 3D printer rental outlet at the strip mall.
Or maybe the government "assassins" would just synthesize matching skin samples in the FBI vat labs.

ETA: To clarify, the latter definitely doesn't exist. No, sir.