Hehe, super funny thread. Basically: buying/building is almost always a lifestyle - and therefore emotional - thing. Just read Kommer's 'rent or buy', that's pretty clear.
So you're approaching it from the wrong side. There's no seller system. More than that, most private sellers are pretty irrational when it comes to price and also finding customers.
If you want to proceed systematically, you have to structure systematically what you can control. The search results are not in your power, only the search process and the search parameters.
You can bring a system into that: search parameters are easy: clarify financing framework with the bank and get a pre-approval, form an exact desired profile and narrow down the location. Many can run beside each other if you don't know what you want.
That way you have defined what you're looking for. And you can filter the background noise.
Which brings us to the second point, the search process: a few things here about why the internet is still enough for everyone who doesn't drink beer with the municipal council in the evenings.
Every private ImmoScout ad automatically goes to eBay Kleinanzeigen. Just like every mobile.de car ad. There's a deal in the background. Then, in Germany there is the company Immobilien markdatenbeobachtung IMV. They search daily (sometimes manually) all sources for ads, prepare them and sell them to platforms like eBay Kleinanzeigen. Of course there's a time delay, sooner or later almost all end up on the internet.
That means for your search process: create filters and set up real-time alerts on all major and regional platforms. And then be fast in contacting, immediately wave the financing approval and say yes I'll take it, go there immediately and act as if you want it right away - you can always cancel until the notary appointment.
The last possibility is to throw your profile at realtors. They often have access - through existing customers or recommendation - to properties that are not yet on the market. If they can save the 300 euros for the Scout ad, they won't be sad.
In summary: the only thing that can be systematically designed in the search is the seeker themselves. I wrote myself a script that immediately sends me a link by email when a new property is on my search lists. The server scans the site every 5 minutes.