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).