You were not involved in the transaction. You had already commented in the A_B post, proving you were aware of the post and thus required to abide by community rules. Considering the nature of your shout, I should have given you a temporary suspension from the community, rather than just a warning.

As I said, best you can do is ignore and block. The less acknowledgement you give him, the more boring harassing you becomes. Unfortunately, there's nothing else we in A_B can do for you, as you do not have an art transaction with this person.