This is detailed in Script Commands
The official example:
<map name>,<x>,<y>,<facing>%TAB%shop%TAB%<NPC Name>%TAB%<sprite id>,<itemid>:<price>{,<itemid>:<price>...}
So to sell a few potions:
prontera,100,100,5 shop Martha 123,501:-1,502:-1,503:-1,504:-1,505:-1
You can also look in npc/merchants/shops.txt for more examples.
Read the manual, that's what it's there for..