Back when we were trying to sell our condo, I was surprised how many sites they were to sell it on and how inefficient most were. We were trying to sell the condo ourselves and non-Realtor didn’t have access to MLS or sites like Zillow & Realtor.com at the time. Because of that we were left to use a site where users entered their own sale data. Where we were trying to sell was near a hotspot with a shopping mall and a direct bus line to our city downtown so it was well located. However, there were a lot of other condo for sale in that area, and the price fluctuated greatly. So, I thought maybe I should make a mailing list tailored to that hotspot.
How to get subscribers? Created a landing page that focus on that niche hot area (condo near NAME OF THE SHOPPING MALL for sale or condo for sale near NAME OF THE BUS TERMINAL).
How to get data? We were living there so that was not too hard. We could go around and enter them by hand. Probably doing some sort of aggregation of other services would be more sustainable.
How to keep subscribers interested? When they first sign in, send them a survey to know what they are looking for and prioritize those types of properties. Send weekly tips post on what to look for, how to do better search, etc.
How to make money? Well if we sold our condo with that we would have made more money than if we used a real estate agent. Just for a 200k condo, the agent commission will be 10k, you need to do a lot of ads word to reach 10k. You can do a subscription where user selling their property can pay to be promoted or to stay on the list after X days.
In the end, we ended up using a real estate agent, but if I created this earlier I think it would have worked. And if it works for one hotspot you can expand it to multiple spots.