make a limitation of obtaining gold through selling by NPC and trading and the limitation increase by level.
for example: level 1, 1 gold
level 2 character, increase by 1 gold
and so on.
this way, it will discourage players from buying gold from third-party as they cant obtain anymore when they hit the limit. when gold hit and rise above the limit quota for that character, the remain gold will be send through the mailing system that can only be obtain on the following day when the quota refresh.
this could also helps the economy in the server.
this will exclude auction house system where else will not be affected by this gold quota system, and therefore encourage players to use AH more often.
this can also reduce amount of scamming activities.