They only accept certain things and they prefer some things more than others. I suggest you go to the "Nanban Trade" page and click on the mini trade good icons to see what's acceptable to them.
Blake.C 09-24-2017
Portuguese players will be able to buy these in Viana do Castelo once its development level reaches 71k (currently at 64k as of this post). As it requires investment, only those players will be able to buy them...