I think the best version of this would only annoy me slightly. Similar to the achievements window popping up when I log on. I check my retainers once per log in, or multiple times if competition on a certain item is heavy so I don't even really need to current MB notifications we have now.

I think I would rather get notified if something gets undercut so long as the item you put up currently has the lowest price, meaning you will only get notified about it once until you change the price.