I was wondering if localization team could shed some light on why these NPCs were given such an odd name.

This seems like a case of Japanese not knowing what word to use so we ended up with this odd translation.

Retainer to me sounds like something that an orthodontist puts in the roof of your mouth.

I'm thinking that perhaps they should be called something else to better reflect what they actually do. If I was a new player and someone was talking about retainers, I can't honestly say that I would know that they were referring to something you use to store and sell items.

Some possible better words I can think of to describe them would be:
1. Servant
2. Caretaker
3. Treasurer
4. Merchant
5. Personal Vendor