I'm not talking about directly ingame. People sell their accounts, items, and gold to others frequently.

You either buy the whole account or you buy it and then they send you the gold/item ingame.

There's a lot of communities, forums, and even businesses that revolve around it. It's a huge market.

Many games it's not technically banned, just not supported due to payment processors not allowing it.

Some games it may be against the rules, but it's hard to enforce.

https://www.g2g.com/

https://mmoauctions.com/

https://www.mmogah.com/

https://www.mmobux.com/

https://odealo.com/

There's hundreds of sites like that, some for niche games, some more general.