This is very simple. Find a game website that you are familiar with and you can trade from their home. If you don't want to do this, you can also choose to trade with the player through the platform of others, but in fact, it is open and transparent, see you. Personal choice.
The POECurrency.com/ that I know has such a trading platform. In fact, they also do trading in buying and selling currencies. I personally feel that the effect is the same. And on this platform, trading is safe and secure, the most important is more convenient