When we sold our first property I was surprised how many websites they were to submit your property. Even after hiring a real estate agent, he suggested adding the house to all the free sites we know. This is to increase the number of people that will see that the property is on the market.
At the time, I even knew somebody who recently built a site where you can put your house for sale. He implemented an option to migrate your listing from one of the more well-known site to his site. That got me thinking, why not do the reverse? Basically the user fills his house information on one site and your service exports it to multiple services.
You could have many tiers, a free tier for free service, a paid tier for paid service and you make a cut through affiliation. Example: our local newspaper had a web form to post your property on the weekend editions for 40$. This service could do it for the same price from our own listing and get a cut through an affiliate link.
Sure nowadays most people zillow, but you want more eyeballs on your property if you want people to know it exists. What better place than a new paper with limited listing than a place where you compete against all listings.
Real Estate Agent Websites
Facebook: marketplace: https://www.facebook.com/
For Sale By Owner: https://www.forsalebyowner.ca/
In your local journal:
example for Montreal Micasa