They create the list of items sold in the shop. From script_commands.txt:
*npcshopadditem "<name>",<item id>,<price>{,<item id>,<price>{,<item id>,<price>{,...}}};
This command will add more items at the end of the selling list for the
specified NPC shop or cashshop. If you specify an item already for sell, that item will
appear twice on the sell list.
The function returns 1 if shop was updated successfully, or 0 if not found.
NOTES:
- That you cannot use -1 to specify default selling price!
- If attached shop type is market shop, need an extra param after price, it's <qty>
and make sure don't add duplication item!
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*npcshopdelitem "<name>",<item id>{,<item id>{,<item id>{,...}}};
This command will remove items from the specified NPC shop or cashshop.
If the item to remove exists more than once on the shop, all instances will be
removed.
Note that the function returns 1 even if no items were removed. The return
value is only to confirm that the shop was indeed found.