How to do and work with Trade List (Not Multisell)

Path: data/merchant_buylists.xml

For example, consider a standard fitting room at the NPC id 30166
Bypass:
[npc_%objectId%_Buy 50|Buy Dye]

and we see the bypass with the id of the trade list 50

Go to data/merchant_buylists.xml

Where 50 it is shop="50"
[npc_%objectId%_Buy 50|Buy Dye]

markup="15" - max castle tax

    <tradelist npc="30166" shop="50" markup="15" >
        <item id="4445" name="Dye of Strength Str+1 Con-3" />
        <item id="4446" name="Dye of Strength Str+1 Dex-3" />
        <item id="4447" name="Dye of Constitution Con+1 Str-3" />
        <item id="4448" name="Dye of Constitution Con+1 Dex-3" />
        <item id="4449" name="Dye of Dexterity Dex+1 Str-3" />
        <item id="4450" name="Dye of Dexterity Dex+1 Con-3" />
        <item id="4451" name="Dye of Intelligence Int+1 Men-3" />
        <item id="4452" name="Dye of Intelligence Int+1 Wit-3" />

also you can add own price by adena or time limit for items.
Example:
Limits:

        <item id="8945" name="Spellbook: Celestial Shield" count="1" time="60" />
        <item id="8946" name="Spellbook: Invocation" count="1" time="60" />
        <item id="8890" name="Spellbook: Summon Attract Cubic" count="1" time="60" />
        <item id="8891" name="Spellbook: Aura Flash" count="1" time="60" />
        <item id="8909" name="Blueprint: Summon Swoop Cannon" count="1" time="60" />

Custom adena price (by default price getting from data/items):

        <item id="8945" name="Spellbook: Celestial Shield" price="1000000" />

Thus, you can write your trade list to your NPC.