I find the auction house as well as the player trade shop very good ideas.
Most important any of those 2 features will make most spam in channel chat go away, massively reduce the time spent on selling/buying pokemon and make it possible for people to buy from everyone, not just from people who are online when you are.
I would be okay if any of these 2 features was implemented, but i personally think it would be best if there was both. An auction house for normal trading as well as a player owned shop which can be accessed by clicking at the player which would encourage personal trading which is more fun in my opinion.