The original post was an idea for a buyer to pay money for a list of houses for around 20% below market valuation. That leads me to think a proper valuation by a charterd surveyour or similar, which will take into account the condition of the building, etc to come up with a proper valuation. Otherwise, I may was well pay for a set of tarot cards to give me a fair price.
As a seller, why would I want to list a property on this service at c. 20% below what I could get with, say being on the conservative side, a 2% fee (usually a lot lower) at no up front fee (unless I break a tie-in clause - which is fair enough)? The only logical reason I can think of is that I am a distressed seller... whether that is because of the property or other debts that will consume the property and I need a darned quick sale darned fast. And you allude to that in one of your latter posts.. Yes, there may be someone who just wants shot of it, but will they really sacrifice 20% of it's real market value to get it off their hands? We aren't taling a few thousand pounds here.
There are already services that purport to provide these listings to buyers for a fee... including reposession and off market properties - so do your research on what they are doing and see how you could better it.. Is it just the price? Fine, a seller needing a quck sale will prefer not to have to pay something if they can get away with it.. and the model where a buyer pays looks already to be around:
https://www.google.com/search?client=firefox-b-d&q=distressed+property+for+sale
Also, you will be up against the likes of Property Rescue.. As long as the seller isn't in default, they may use these services to quickly release the property for cash to stave off bankruptcy.. they may pay less than 80% of market value, but the cash is in the account within a week or so from memory (I have no idea of these businesses,but I would imagine they would fall under the same types of ethics as the original payday loans companies).
I would imagine your target market is investors or bargain hunters, because for people buying a home, there is so much more than the fact that they can get the house for a lot below market.. the layout of the house and its scope for being able to facilitate the lifestyle they wish is very important - it's why people often go over their budget into higher risk territory when buying their home.
If you can really get supply where the property is 20% below true market value and not a location average (we are assuming residential, but I guess commercial as well, although that opens up another pandoras box post-COVID), where you are making 20% minus costs for 9/10 of the properties, and say break even on the other 1/10, you will not be short of investors backing you...
On the original idea, as they say in a well known captial raising TV show, "I'm Out"...