This is what I'm doing...
1- C:\opt\l2j\server\game\data\html\merchant\00001.htm (this is my new npc id)
2- I put the npc on C:\opt\l2j\server\game\data\stats\npcs\custom\custom.xml<html><body>Trader Graham:<br>
Let's trade! I'm sure we can arrive at a mutually beneficial arrangement. What are you offering?<br>
<a action="bypass -h npc_%objectId%_Buy 3008400">Buy weapons for Fighters.</a><br>
<a action="bypass -h npc_%objectId%_Buy 3008401">Buy Fighter Common Items.</a><br>
<a action="bypass -h npc_%objectId%_BuyShadowItem">Buy a Shadow Item.</a>
</body></html>
3- And the command //spawn 00001 doesn't work<npc id="00001" displayId="32140" name="L2AT" usingServerSideName="true" title="L2 Arrowtown" usingServerSideTitle="true" type="L2MerchantInstance">
<collision>
<radius normal="11" />
<height normal="22.25" />
</collision>
</npc>
This is an example npc for learn how to do it