I was quite startled by the stickers. If JKR was of a mind to sue for libel, then the burden of proof is on him, that is to say that his stickers are true statement. He cannot claim the defence of opinion since the sticker clearly asserts a statement of fact, rather than opinion.
As to reputational damage, it is one thing tiffing on line quite another to encourage others to parade a libellous statement.
As for it just being a grift, it would be a risky business for him, as an award against him could ruin his business.
personally i think he has lost his mind.
I want her to sue him, because we would see the definition of transphobia explored and dissected in court.
We will see if JKR turns the other cheek, could be interesting…..