The competition (WoW) has an app that does this and more. It's included in your sub. It lets you create far more listings per day (200!) with no mobile currency BS required to do it.
The MB is already on functional servers, as are login and chat. The only extra server capacity is on a frontend to handle requests from the app. $5/month for that is a total farce. If they want to do this, it should be a value added feature for people who are already paying a premium price, considering this is one of the most expensive games on the market already.
This is a shameless cash grab to an insane degree. One listing per day, or two for $5/month, unless I pay extra for special currency? I mean, I know Blizzard's technical know how trumps SE, but there's no excuse for such low limits on the app unless the goal is to try and get people to buy extra currency to use it, which is disgraceful behavior in a subscription game. This is exactly how F2P apps work.