Understanding public key cryptography example
Interesting premise.
Q : &quot;Is the ad encrypted with his private key ...&quot;
A : no, I don't believe so
Edit - To clarify, I entertained the possibility that the author meant to say &quot;private key&quot; in item #4, but once you consider the subsequent paragraph in the source (excerpt below), I don't think the ciphertext that describes the crime in advance is encrypted with public-key cryptography. I reckon the author's protagonist is using the same bytes that are in the public key as the key for a symmetric cipher. (In this case, KDF could potentially be HKDF, PBKDF2, Argon2, scrypt, or even a keyed hash-digest.) eg.
broker_pk = PUBLIC_KEY( secret_key )
ciphDetailsOfCrime = E( KDF(broker_pk) , EvilPlanToMakeCelebsEvenMoreFamous )

eph_cli_pk = PUBLIC_KEY( ephemeral_client_secret_key )
kRequestForQuote = DH_or_ECDH( broker_pk , ephemeral_client_secret_key )
ciphRFQ = E( kRequestForQuote , DetailsOfNewTarget )
msg = eph_cli_pk || ciphRFQ

(This is a contrived example that shouldn't be used, especially if conspiring to commit murder and/ or acts of terrorism.) My original answer continues ...
It may be that the author meant to say &quot;private key&quot; in list item #4, however, I suspect the author is thinking about using the public key, posted on billboards, as a symmetric key (or password to be used in some key derivation function - aka. KDF) to decrypt the prior description of the heinously foul act of murdering film celebrities en masse ...
The paragraph that follows is:

&quot;You have now made sure that everyone in the world has, or can get, your public key – and knows that it belongs to an organization willing and able to kill people. Once you have taken steps to tell people how to post messages where you can read them, everyone in the world will know how to send you messages that nobody else can read and how to identify messages that can only have come from you. You are now in business as a middleman selling the services of hit men. Actual assassinations still have to take place in realspace, so being a hit man still has risks. But the problem of locating a hit man – when you are not yourself a regular participant in illegal markets – has been solved.&quot;