as many of you have been questioning, zinc ore CAN be bought by an NPC.
the goldsmiths guild in bastok sells it on ONE DAY a week only. im not sure if the day changes, or its set and if it is set im not sure which day it is. it usually costs about a little less than silver, 200-250 ish. for those too lazy to farm for the tenshodo quest come here. if anyone knows which day it is exactly please do tell.
i have seen it on sale twice so far so i have proof it is sold.