It turns out that merchant inventory is within a chest hidden outside the map. Watch me put 2.4 million gold in Niranye's hidden chest and then watch her available inventory gold increase
Every Hidden Chest in Skyrim* - Other Games - 343Industries Community Forum
Skyrim mod adds hidden keys to find for all those locked chests | Rock Paper Shotgun
the elder scrolls v skyrim skyrim dragonborn - Where is Majni's merchant chest? - Arqade