Would the said players lose anything meaningful if they lost their accounts, or are they just using disposable accounts to show off? If they're just disposable accounts, they may end up making new accounts to start over.
Some people have so much money they don't care if they waste it. Ultimately, account bans aren't going to stop that type of player unless they've managed to obtain something extremely rare on their account that they're unlikely to obtain again just by spending money.
Most players aren't in that position. They might have use RMT to purchase something the first time but take it away and they probably won't want to take the risk a second time.
A lot of the price is in what other players are charging for the tomestone crafting material. Crafters don't control those prices - the players with tomestones do.
As an example, the marketboard prices on my server for the tomestone materials jumped from an average of 3k gil each to 12k gil each last week as players started using the tomestones to get their first stage relics instead of buying crafting materials to sell. The cost of those materials along with all the rest of the materials needed ends up passed on to the buyer. Weapons and chest pieces need 10 of the tomestone materials so that's already about 90k more it's going to cost the crafter to make the item.
That becomes another factor in the demand for RMT services - what players are charging for the items they want to sell. When players are setting prices too high, a lot of the buyers turn to RMT to be able to afford those items. RMT sells the gil to the buyer, infusing more gil into server economies via both bot farming and taking gil off compromised accounts that may have been sitting unused for a while. Because there's more gil, sellers feel safe increasing their own prices so they get richer. And the cycle begins again as more players decide they must turn to RMT.
The cycle exists because there is no penalty for buying from RMT. If there was a meaningful penalty, few players would turn to RMT and the amount of gil entering the game economy wouldn't increase as swiftly. Because players would have less gil, they wouldn't be willing to pay as much. Crafters would start lowering their prices sooner to make the sale because the players able to afford the original prices would already have what they want.
It's one reason why I have no problem undercutting by large amounts or even crashing markets on some items despite other sellers sometimes attacking me for doing so. I'm bringing prices back down into a reasonable range. It leaves players with more gil to spend to buy the other things they want from other sellers, reducing pressure to resort to RMT to get the things they want.
But still the core problem ultimately lies with those willing to buy from RMT for instant gratification instead of putting effort into earning the things they want, whether that is doing the content that rewards the item via their own efforts or earning the gil through legitimate game play to buy it.