It's a good idea, but it would take too much time and resources to develop.

@Bopp

Consider how much this game taxes even a desktop computer, probably due to being written in a high-level language/library without much optimization. To make the game run efficiently on a weaker device, it would have to be completely rewritten.

That's actually no problem at all. The taxing part is the 3D graphics. An auction house/energy market client can do without this. (Almost) All you need for this is in the Narya library. The actual problems lie elsewhere:

• You always need a knight to access the AH (obviously, otherwise you can't access "my bids"). This leads to the awkward situation of you being "logged in", but not responding to chat (or at least having to explain to your friends that you can't do anything right now because you are on your phone).
• The game usually idle kicks you after 10 minutes or so, simply because every logged in knight uses resources on the servers. The whole idea of an AH app is to be constantly up to date. That means you potentially have hundreds, if not thousands of knights in limbo (ready room), constantly "clicking" the reload button on the AH.
• To log in, you need credentials. No problem for players who use the standalone client, but steam, kongregate, etc. users are left out.
• OOO has nothing to gain from supplying a market client. It's just another piece of code that needs to be written and maintained. That costs money withotu returning any (unless this client somehow had a way to easily spend cash).
• The whole idea behind MMORPGs is: the player is (part of) the content. You are suppose to log in and spend time on interacting with others. A stripped down client runs counter to that.
• EDIT: You get an outbid notice while in the middle of a run. Since you can't return to Haven at the moment, you get the idea to check your bid(s) from the phone - whoops, best situation, ever :-D. The mobile client would naturally log your knight in a secod time and by doing so, disconnect your desktop session. Now imagine the fun you'd have if you set your phone to regularly check the market and totally forget about it. I bet, Game Masters would totally hate the volume of "Why does SK constantly d/c" tickets.