Try:

<?xml version="1.0" encoding="UTF-8"?>
<npc name="Archer Seller" script="data/npc/scripts/archer.lua" walkinterval="2000" floorchange="0">
<health now="100" max="100"/>
<look type="137" head="40" body="96" legs="95" feet="96" addons="0"/>
<parameters>
    <parameter key="module_shop" value="1" />
    <parameter key="shop_buyable" value="small stone,1294,3;arrow,2544,3;bolt,2543,4;bow,2456;crossbow,2455;spear,2389,10" />
    <parameter key="shop_sellable" value="spear,2389,3" />
    <parameter key="message_greet" value="Oh, please come in, |PLAYERNAME|. What do you need? Distance weapons? I sell lots of them."/>
    <parameter key="message_farewell" value="Good bye."/>
    <parameter key="message_walkaway" value="Good bye." />
    <parameter key="message_sendtrade" value="Here you go." />
    </parameters>
</npc>

Ainda não... o erro persiste.

<?xml version="1.0" encoding="UTF-8"?>
<npc name="Archer Seller" script="data/npc/scripts/default.lua" walkinterval="2000" floorchange="0">
	<health now="100" max="100"/>
	<look type="137" head="40" body="96" legs="95" feet="96" addons="0"/>
	<parameters>
		<parameter key="module_shop" value="1" />
		<parameter key="shop_buyable" value="small stone,1294,3;arrow,2544,3;bolt,2543,4;bow,2456,400;crossbow,2455,500;spear,2389,10;" />
		<parameter key="shop_sellable" value="spear,2389,3;" />
        <parameter key="message_greet" value="Oh, please come in, |PLAYERNAME|. What do you need? Distance weapons? I sell lots of them."/>
        <parameter key="message_farewell" value="Good bye."/>
		<parameter key="message_walkaway" value="Good bye." />
		<parameter key="message_sendtrade" value="Here you go." />
    </parameters>
</npc>

Não use o arquivo lua, configure apenas por esse arquivo xml desse jeito. Para adicionar novos itens, use ;nome,id,preço (não esqueça de posicionar as aspas (") adequadamente)

#Editado, consegui, rs. REP+

Tópico movido para a seção de dúvidas e pedidos resolvidos.