content of data/multisell/1.xml:

<?xml version='1.0' encoding='utf-8'?>
<!-- Non-grade items exchange, just for education -->
<list>
  <!-- [leather_shirt] for [shirt]+[adena];2100 -->
  <item id="1">
    <ingredient id="57" count="2100" enchant="0"/>
    <ingredient id="21" count="1" enchant="0"/>
    <production id="22" count="1" enchant="0"/>
  </item>
  <!-- [wooden_breastplate] for [leather_shirt]+[adena];5400 -->
  <item id="2">
    <ingredient id="57" count="5400" enchant="0"/>
    <ingredient id="22" count="1" enchant="0"/>
    <production id="23" count="1" enchant="0"/>
  </item>
</list>

file name means list id, so 1.xml contains multisell list 1, 23.xml contains multisell list 23

each list entry decribed in <item id="">...</item> entry contains few fields: few fileds of type <ingredient> and 1 field of type <production>

<ingredient> filed contains info about items what will be removed for player inventory <productction> filed contains info about item what will be placed to player inventory

so entry:

  <item id="1">
    <ingredient id="57" count="2100" enchant="0"/>
    <ingredient id="21" count="1" enchant="0"/>
    <production id="22" count="1" enchant="0"/>
  </item>

means: player requires 2100 adena(itemId:57), 1 Shirt(itemid:21) with 0 enchant level, and he got 1 Leather Shirt(itemid:22) with enchant 0

to call mulstisell from .htm:
...
<a action="bypass -h npc_%objectId%_multisell 1">Multisell list 1</a><br1>
<a action="bypass -h npc_%objectId%_multisell 2">Multisell list 2</a><br1>
...