Not only is it not bannable, but is actually considered a legit transaction between players from the devs/GM point of view.
(Eg. Post from 2017 with GM being quoted as saying such:
https://forum.square-enix.com/ffxiv/...=1#post4500631 )
#28 on that thread:
GM:"I see, according to the logs they seem to only be selling runs for Gil and that they have it listed cross-world by default."
GM: "Selling runs for Gil is not against the terms of service."
GM: "I feel that I have everything I need to begin my investigation. Before I take my leave is there anything else that I may be able to assist you with today?"
I ask if they can request the advertisers to uncheck the option that has the listings being shown across the data center.
GM: "No I can not"
GM: "Players are allowed to do this and it would be interruption of their game play if I were to make them stop."
Me: Even though it's across the data center? (paraphrased)
GM: "That's correct."
Me: How are they receiving gil across servers? (paraphrased)
The GM then ends the conversation.