Been getting this since late last night whenever I try to sell through the app. Only happening on my lvl 90 - other characters are ok.
Multiple uninstall and reinstall of app has not fixed it.
Saw an old thread about the same issue when 5.1 dropped http://us.battle.net/wow/en/forum/topic/7200052586
Thread made me think that maybe something 5.2 related is in my bag and causing it. Only thing I had in there that was "new" was the Kirin Tor Beacon. I dumped that and still getting the same message. There was a blue post in the thread that mentioned a mobile server patch. Maybe need one for 5.2?