I have been considering for a while to start a portal for office space leasing, in my country there is only about four websites that do this and just two of them are true competitors. But I feel like I would have a good chance to make it, but as Always in business you got to make a market research and knowing that there is a demand for your service. I was thinking that I don't want to call every real estate company and ask them about the demand, I'm just going to start a portal and then try to putt all there ads for free, then when I have a good amount of visitors. Then I can start charging for my service, does that sound like a good business plan? Of course a very simplified one.
It sounds like a decent low cost plan. As long as you have the time and can afford to go for it. Then once (hopefully) the site is successful and getting lots of hits you can present this to the real estate agents and start charging. But often these markets in each country are dominated by one or 2 websites, so you may have your work cut out getting your brand out there against these other guys