Hmm, from my imaginary buyer's point of view I don't see how much it will cost to add you as an intermediary. Fixed rate? Percentage? I mean I signed up with my Google account but then was asked to give my phone number, and... yeah that's about where my own drawbridge tends to be commonly raised, especially when I know so little. So I aborted. I mean my instinct said that maybe if I were really interested in a property I could just, like your site mentions, send a message to the physical address and then if that didn't work maybe come back and give out my personal number, maybe even after looking online for the owner. The site looks nice btw.