Dear SE
You have a problem, and it will not diminish by following the well worn path you are currently on. As proven time and again, you can ban thousands of bots and they just return. Now, I am not going to say, work harder at that, I am not even going to say give up on it. But I am going to say, you need a different approach because the bots simply will not give up.
As crime shows say... follow the money, or in this case, the gil.
Bots exist for one reason if you were not aware, and that reason is to generate gil to sell to players. I doubt this is news to you.
What you need to start doing , is to inconvenience the buyers. 3 day vacation for first time offenders, maybe a week for the second and the third time, gone. Perma ban.
This is something that can be, if not already, tracked on YOUR end, there is no need for client side code or spyware. This is you, you having code which I am sure already exists on your side of the software. If someone suddenly gains a lot of gil, it should set off a flag for a GM to look at. Lets face it, no one in their right mind spends 5 million gil (or more) on a level 30 item when every other listing is for 1500.
Penalizing the buyers works, I know. Years ago they used to do this in games, and it is a deterrent if you enforce it. You already say its against the ToS.. so enforce it. No one is going to want to lose their in game progress buying gil. So, go after the buyers, once that gets around, and that it is enforced, with a potential perma ban at the end, the market will dry up and the bots will decrease.
There is no downside to this for you. You are enforcing the ToS, you are fighting the bots, and in the end, its a win for you and your reputation, and for the players. You also do not have to add any clientside code that people will complain about, this is all something you can track in your database.
If you do see this, or get this approach, I think it will go a long way to combating a pervasive online gaming problem. Thank you for your time.