Just choose an NPC in Junon Polis, check its list-sell references (4 columns in LIST_NPC.STB) then go to the correct row (that matches the ids in the columns) of LIST_SELL.STB. Add the items there then add them into sell-data.csv in your server.
Sorted. The items will appear in the NPC shop.
The tutorial is easy enough to follow isn't it?
If you want to use a custom NPC then first you have to make that NPC appear and that is a totally different subject. There are tutorials for that too but i didn't write them.