What is a trading greylist?

why it happens everytime i get unbanned?

It's a list of players who are unable to trade.
There are some actions that may get you into it, buying items from shop with money being the most common one. Contact an administrator if you want to know why you got added to the list.

but admins also don't know

I'm having this same problem, but all I did was buy something from shop with $$$. I've gone through 6 days of waiting.

he has.