There must be something behind the whole free to play thing tbh, if you think people really run a private server for free because of their passion for the game, you are wrong, it's a business like every other private game server, just different business models. Private servers can't charge players monthly fee, so they have to get money either from donation or other ways, no one does this kind of heavy duty for free, it's a job for a team.

What would you do if you are the owner of the server? What I can think of maybe sign contracts with those gold selling sites or gold sellers to share profit? or maybe the owner owns the site and they are the sellers since GM can create gold with commands? They ban those gold sellers without signing contract? Who knows.

I've heard that private game servers in China are the worst, players' computers are compromised and being sold as zombie PC for DDoS attacks, hackers running those private game servers and people don't really know who they are behind the door.

Why server keep getting DDoS'd? because players all came to play on this server, other private servers lost a lot of players meaning they lost income(donation), this always happens in other games' private servers from my experience, when a new server launches, the other private servers attack it to make it unplayable until players come back to the server they used to play on.