and even if you afk and leave your shop open ( i do sometimes), people might whisper you later for similar items and that creates interaction. This happened to me in a diff game on multiple occasions and one of them led to me joining their guild. Fast forward 5 years later now were all playing this game together. To me, the social aspects of an mmo is what sets it apart from other game types. Personal shops should be part of this aspect, to have the OPTION to be more personable and possibly realistic with how they choose to play their character.
p.s. having a retainer stand in for you even if your logged out in one particular area is a great idea dont get me wrong, but then why would people use it at all when the MB is worldwide and sells when you are offline too? I am sure it had some great aspects but it does not sound like a 'personal shop' more like a unique system for an auction house.