Maybe try changing the gold to 0!
Inside your npc file open it with notepad++ and change the gold, Example
<parameter key="shop_buyable" value="amulet of loss,2173,25000;swift amulet,2196,50000;desolations amulet,2138,1000000"/>
Change it to:
<parameter key="shop_buyable" value="amulet of loss,2173,0;swift amulet,2196,0;desolations amulet,2138,0"/>
And it should be free!