In a certain Korean MMO I used to play that “retired”… They banned the currency trade of over X currency.
Then what (legit) players did, is trade valuable items that matches the other’s desired amount or just buy a certain X item from X named player on the AH to entirely settle the matter. Includes AH tax to buyer too so seller isn’t even impacted by it.
We see this happen from other recent MMO’s like New World that, while they did not ban the currency trade; they just inserted too little currency into the game so there was a shortage of currency and people were doing like what they were in the Korean MMO: They traded items for items only and severe deflation occured on existing currency.