Yes, I like the idea of having messages sent to you indicating when something has been sold. What would be nice is a history that gives you a list of what you sold, when you sold it, and for what price. If possible, let me graph it.

Give the retainers the ability to level up based on how much money they make (or other factors). Put in specific algorithms in place to prevent gaming of the system (e.g. putting up a whole bunch of items and having a friend buy it all at once and give you back the money)

Upon level up, a retainer's appearance can change, it can have increased item capacity, it can have increased bazaar capacity, it can sell things for less tax, etc.