Ineed to know why dis chinese traders demand payment for their good in gold?

In order to have a trade, both partners must have goods that the other partner wants. And Europeans didn't have anything that the Chinese wanted; so they gave them gold instead.

Also, another factor that probably came into play, was when the Europeans first came to trade with the Chinese, the Chineses saw them as barbarians. Chinese also thought of themselves as the "Middle Kingdom," a place that was superior to the rest of the world. 

Therefore, they had no interest in anything that the Europeans tried to trade with them.  They would only accept gold and silver.

This later became a problem for the Europeans because this drained their reserves of gold and silver. 

This is why they eventually decided to start trading opium to the Chinese in exchange for silks, porcelains, and tea.  This led to a great deal of conflict and, eventually, the Opium Wars.

Existing to a lot of people