I think I've encountered a similar app in the past that was working with Twitter API to do something similar (don't even remember who they are). Irrelevant to the point anyway.
Anyway, here's my initial thoughts and PLEASE take it with a massive grain of salt as I don't speak for everyone else...
This idea seems way more appealing to businesses than it would to users i.e. I need to hire a ton of Stanford students, announce job posts, etc... When I was in college, the marketplace argument sounds great in theory but eBay, half.com, and craigslist are not too far away (plus rentals online now that removes reselling problems). If I want to resell, I want to get mass market reach, not just local (whatever is best).
I can also see this as a potential app that rings up too much spam, too many announcements (shouts). I can't find an immediate need for why I would want to make announcements so often and more so, I can't find a reason why I would engage in the app frequently because of such. I can see some power users that may want to do so (for things you've listed), but on a whole, I can't speak for others but I don't see a strong compelling reason why I would want to "shout" anything generally. Particularly to such a niche demographic (unless again, I was a business or something similar in nature where I have a reason to pitch to users, but this doesn't provide a good reason for why users would want to engage).
On the surface, I don't seem to have a problem this app is trying to create a solution for, and there is no strong compelling reason on why (even after the pitch) I would want to use it or continue to engage frequently enough (daily active).