This is a classic chicken and egg problem. We need both users to be active in order to be successful, therefore requiring us to target both user types at once.
A lot of great advice here.
On top of that, here's a brilliant article by my friend from a startup called UniPlaces - I hope you'll find it useful! http://albuquerque.io/what-we-did-to-grow-student-marketplace-uniplaces-by-1000-in-2014