I know Blizzard is kind of a touchy subject at the moment but they invented the perfect way to combat RMT: The WoW token. The WoW token can be bought with real money and auctioned off ingame for gold and the buying player can redeem it for subscription time.
In this particular case, a PSO2 token it should just be vendorable for tons of Meseta/N-Mesta and should work with both PSO2 and PSO2NGS.
Restricting F2P players from selling on the market did not fix the problem. Not unlike with illegal copying of software (which is neither piracy nor theft), the problem is one of availability and ease of access and if anything, you actually made the problem WORSE! That is, the only way to combat illegal RMT is to provide a legal alternative.