My opinion is, complicated.

I have no idea what the background these botters come from. (An individual living in the US/AUS/EU who is living in the basement of there parents house, or if they're from a nation that has weak currency, and $1usd=100$usd;equivalent in there country). Nor do I know what there actual sales figures look like irl. Such as, how much gil people buy per weekday/weekend/on holidays. So as an example this is what I was imagining in my head the other day:

1.) 1 mil gil = roughly $2.88 USD
2.) Selling 10 mil per day= 28.80USD
3.) Per month they would average $510 USD

As a closing comment, there's no real way to combat these individuals. They're offering a product that people are willing to trade irl currency for make believe currency. It's relatively simple to make a new account. So no matter how often you ban then, they can keep populating.

And as a business, I would bet 100$ they're smart enough to have a C2 element that stores there gil/sells the items. And those characters aren't getting banned because people don't report players who just sit at the MB/by the mailbox.