That one's impractical. I sell Oak Branches. I put them up in stacks of 99 to start with, sure, but people don't need 99 Oak Branches. They just buy them a couple at a time. What are you proposing SE does with my retainer once someone buys a single one of my Oak Branches? Does it get kicked out of my retainer's bazaar? If that was the case, I'd never even sell my Oak Branches; I'd just NPC them all since I sell them for only slightly more than what an NPC would offer me.
On the other side of that complaint, do you know how impossibly long it would take to acquire 99 Vampire Plants to sell? Alchemists would never be able to find half of the random junk they need for medicines if people were forced to acquire a full stack prior to selling anything.
I'd be fine with the 1 retainer per city thing though... that's what I do currently, because I like to be able to have a retainer accessible from anywhere.