Margarte is a Nord merchant in Leyawiin. She can be found either in the Great Chapel of Zenithar in Leyawiin, in her house directly across the street from the main entrance to the temple, or just outside the city collecting alchemical ingredients.
Interactions
Drunk and Disorderly
Modryn Oreyn sends the Hero to Leyawiin to investigate what has happened to three other Guild members. Apparently, they have been drawing a lot of negative attention towards the Fighters Guild and causing trouble at the Five Claws Lodge.
Mercantile Training
Margarte will refer the Hero to Palonirya once their skill level reaches 70. Palonirya runs Divine Elegance in the Imperial City.
Trading
Margarte states that she is interested in buying Ogre's Teeth and Minotaur Horns. She will buy them in bundles of 5 for 50 
.
Routine
She is available for Journeyman Mercantile training whenever she is not sleeping (12:00 AM to 6:00 AM), eating breakfast (6:00 AM to 8:00 AM), collecting ingredients (12:00 PM to 6:00 PM), doing Sunday worship (Sundays from 8:00 AM to 10:00 AM), or eating dinner (8:00PM to 10:00 PM).
This means shes available for training most days from 8:00 AM to 12:00 PM and again from 6:00 PM to 8:00 PM, and from 10:00 PM to 12:00 PM. She will most likely be at the chapel during those hours.