Honestly a great start up would be a review system for house listings.

Users can rate how accurate the description was, the real life flaws and even upload their own photos.

Side note: last time I looked for a house I really wasted 95% of my time because every house had one unique major flaw that would have made me not even bother going to see it.

Announcers get very touchy with listings data, so even compiling listings from multiple sources is hard without getting cease-and-desisted. Then, realtors will certainly flood competing announcements and post fake reviews. It's an aggressive market.