You can do this using an itemshop type NPC.
-%TAB%itemshop%TAB%<NPC Name>%TAB%<sprite id>,<costitemid>{:<discount>},<itemid>:<price>{,<itemid>:<price>...}
<map name>,<x>,<y>,<facing>%TAB%itemshop%TAB%<NPC Name>%TAB%<sprite id>,<costitemid>{:<discount>},<itemid>:<price>{,<itemid>:<price>...}