I would suggest adding more value than just a website.

About a month ago, there was a bit on TV about a Melbourne couple that had started up a For Sale By Owner company, offering a complete service to potential sellers.

They supply assistance, a check list of things to do (get 3 realtors to look at your property to price it, make sure you have a legal contract before advertsing etc.)
They supply a large professional coroplex For Sale sign, web site with pics etc.

The complete package goes for $995.
Don't underestimate the fact that the average person is clueless and does not know the best way to sell their property. An agent is generally the path to least resistance.

The story was news worthy enough to make it on Prime Time TV.

It is always better to offer a complete package.
It dramatically improves the value of your product, allowing you to charge more.

Most people that are thinking of selling look thru local papers to price similar properties etc, so this would be the ideal place to place an Ad.

Also, if you can add value that a real estate company can't easily do without getting in outside help, such as Virtual Tours, that's a bonus.

I just sold my property without an agent and created my own virtual tour. It generates interest to passerbys when you have the URL on your For Sale sign.

Depending where you live, it can be extremely difficult to sell Realtors the idea of joining your portal, if you go that way.

See this forum at Slashdot where they discuss selling a listing database to realtors.

http://ask.slashdot.org/article.pl?sid=03/08/06/2043214&mode=thread&tid=126&tid=156

To list realtors on a portal, in my opinion, will depend whether you live in a large city that has a competitive market. In Sydney you would not have a hope in hell of creating your own Real Estate portal that is viable.

The Real Estate agents are virtually all locked in with major portals like http://realestate.com.au
It is too late for that.

If you would like a few pointers in Virtual Tours as an extra bonus for a Sale By Owner website, feel free to drop me a line.
Check out my site too if you like:
http://insideouttours.com

Good luck.

Tom B.