Quote Originally Posted by FoxyAreku View Post
Retainers don't act as a market board, from my understanding. They just sell what they are currently selling.
I see it as a fun feature to have a shop of random goodies that is constantly changing on your lawn.
From the patch notes:

Once a retainer is placed on estate grounds, items that they currently have listed for sale on the market can be purchased by anyone who speaks with them. The retainer's sales history (up to 20 entries) can also be viewed by other players.

Sounds to me like they are nothing more than another way to interface with the MB...