You're in a bit of a catch 22.
- You need real estate listings on the site in order to attract buyers/renters.
- You need buyers/renters visiting the site in order to attract real estate companies to post their listings.
I agree that you shouldn't wait to launch, but you need to find a way to get a critical mass of both listings and potential buyers and renters as soon as possible. What distinguishes your site from the competition? Why will agents list on your site? Why will buyers/renters come visit your site?
What is the scope of your site? Are you targeting real estate around the globe? a whole country? a specific region?
Would it work for you to focus on a small region to get started? It should be much easier to get a significant amount of listings for a small area and then add other regions later. If your site can be the complete listing site for one small region, that would be a competitive edge over other sites that have a broader reach.
Also, don't be afraid of the "Beta" label. I think average web users are getting used to that and know what it means. Google has proven that millions of people will use a Beta site. They have also mastered the art of using a limited access beta to create huge buzz around a new site. Take Google Wave for instance. I still don't really know what I'll ever use it for, but since everyone needed to be invited to the site, I asked a friend for an invite and went to check it out. Of course, they're Google, so something like that may not work for you, but I think it's something to consider. Does your site have a compelling enough story to launch as a limited, invite-only, beta?