Problem when selling more then one item..

[Burdin]

I've made a NPC which buying Giant claws from players..

My problem is that the npc do only buy the item if there is 1 giant claw in the backpack.. Whenever a player have a stack of the item, the npc won't trade it 0.o? Somebody knows what is wrong?

<?xml version="1.0" encoding="UTF-8"?>
<npc name="Reza" script="data/npc/scripts/default.lua" walkinterval="2000" floorchange="0">
    <health now="100" max="100"/>
    <look type="128" head="55" body="40" legs="33" feet="115" addons="2"/>
    <parameters>
        <parameter key="message_greet" value="Welcome to the southern outpost, adventurer |PLAYERNAME|! "/>
        <parameter key="message_farewell" value="Good bye and come again, |PLAYERNAME|."/>
        <parameter key="module_keywords" value="1" />
        <parameter key="keywords" value="job;" />
        <parameter key="keyword_reply1" value="I'm a farmer of the south! collecting valuable Giant Claws.." />

        <parameter key="module_shop" value="1"/>
        <parameter key="shop_sellable" value="Giant Claws,5930,3500;"/>
    </parameters>
</npc>