I see a lot of posts defending retainers as being a new idea. Nope.
Ultima Online. They also had npcs which you could hire but for the purpose of selling your goods. You placed them around your house and you could have as many as you could afford. The system worked simply because you clicked and the goods displayed. It was very easy to browse. It made buying and selling very immersive and people put up shopping malls, which was kinda awesome.
Everquest...didn't have such a thing when the game launched but they had a system kinda like SE's but it was just a toon converted to a market. They all stood in a bazaar which was somewhat similar to a market ward. Big difference though, in that it was searchable (through a npc) and when the item you wanted was located there was a trail to guide you to it.
Star Wars Galaxies. Also had stores in player housing, similar to Ultima's (same lead dev) but it also had a auction house. Players prefered the auction house, but the vendors in player housing was immersive.
Subsequent games had auction houses. Best ever in my opinion is EvE online. You had to buy and could sell from your home (station) as it were, but you could see what was available elesewhere. you could even buy it remotely but still had to travel to get it.
Yes, a retainer system could be better for immersion, but history has shown that auction houses are the prefered way to go.
Innovation is taking an existing idea and improving upon it, so SE is trying to innovate with market wards, but they are definately not improving on it.
Invention is something totally new.
Just wanted to clarify a few things for the fanboys who think retainers are a new idea.