The ID of your NPC and the name of your html file must be the same. And of course the NPC's class must be L2Merchant, or something like that, if you put it's html to the merchant's folder.
It's much simpler to search for it in MySQL, and of course in this way you don't have to ask again and again every time when you have to find an id. :P
You can add effects to your weapon in weapongrp.dat file, here is the list of effects and a tutorial from -=ANUBIS=- :
http://www.maxcheaters.com/forum/index.php?topic=19647.0
I don't know how to make your own effects, but you can change their textures as well.
Count the number of columns, maybe there is a difference in the version of L2, so for example you can't copy an Interlude row and paste it in a Gracia weapongrp.dat file, because the number of columns isn't equal. So check it and fix ;)