If you're going to build a new portal, start in a niche like spare rooms, aggregate stock from other sites like Spareroom and build up a loyal following.
Once you have a following, allow people to list.
This is what Kangaroom's founder Jinder Sidhu has been quietly doing. Simple, but very effective.
Have a play with the website: http://kangaroom.net - both browsing and listing property is probably the most slick experience of any property website out there.
This is definitely one to watch.