I do believe that the only way to stop gold sellers is to stop gold itself.
Sad truth. Hmm… try “no more trading system (player to player)”.
Create a NPC that sells every item at a price.
Example:
8m for rarest
2m for rarer
500k for rare
50k for common
Whenever players sells these items:
2m for rarest
500k for rarer
100k for rare
10k for common
Monsters drop 1k silver. Bosses drop 10k silver. They rarely drop these items as well.
With this, you can also reduce bots from gold sellers. Can’t stop bots. There will always be bots for whatever reason (many reasons). Try creating a new server that is pay to play. Give these bot users a reason not to use bots with many accounts.
This way - with two servers (one is f2p and the other is p2p), complaining players have options. There are people that are sick with these robots.
Server 1: the server that we are having right now. (with trading system)
Server 2: new server that is with my suggestions.
…
Else, let’s just all accept them, for they will always find a way to be part of the game. People who are playing fairly, I’m talking to you. 
“The only fair in this world is probably yourself. Are you?”
I see GMs are finding ways to stop them, implementing it, sooner it will fail, then implement something new again, and the loop goes on and on until one side decided to give up.
That said, my suggestion will probably fail too.
Just please, fix all the current bugs as soon as possible. These bugs are more annoying than robots.