People are trying to do this in the UK with businesses like Purple Bricks.
Given the aggregators exist (in the UK, Zoopla & RightMove are our Zillow) that's where all the eyeballs are and how people find properties. You can then list your own property there.
Before I started Gitter, I nearly went into exactly this space, but there are a few negatives:
1. People are generally bad at sales
2. People are generally bad at negotiating
3. People are generally afraid of complex paperwork
Sure, the system can help with these things and maybe nobody has nailed this yet because there are a few companies trying in this space and nobody seems to be coming out on top.