Great idea. I also feel like the chat function could be extended to some degrees but it's not the worst thing. I also feel like there should be some way to incorporate voice chat in to this feature.

Micro-transactions could use some work. Right now it's hard to justify having any reason to use coins to do anything. Most people interact with each individual item on their list to update costs or post more than one thing for sale at a time. I think it'd make more sense to have a daily pass and/or hourly pass system as it would feel more fair to the players. Not only would it feel better as a money drop it also rewards the players for jumping in to your app and pulling it up more often.

Add minigames or other functionality to the app that has direct connectivity to the game. If I can triple triad my buddy with my deck, I am all about it.