Sorry for never noticing your message until now. I didn't get any notification about it.

Do you still need help with that?

Sure, I never really figured it out besides selling it lol.

They are mostly used with the rank 2 powers.
The first is by clicking on the castle/city picture of your demesne and selling them there, giving a stats boost to the province.
The second is by crafting, there is a list in the decision in the intrigue menu. These are mostly to make more valuable or rare trade goods to sell for more money or for more stats boost in your provinces.

It took me forever to get what you meant, I had no clue it required you to be IN the trade guild to use them, thank you for that bit of info

If you aren't in the league, gift everything to your trade master if you have one, that's his job. :)

How do you sell stuff to others?

Other characters? They'll ask randomly to buy from you but iirc it was fairly rare. The only pro-active way to sell is to invest in province and attempt to sell it in the market.