The issue with the websites already in use by agents is that the property is almost always out-of-date.

Agents are not incentivised towards keeping listings fresh. It's better for them if they can tell the person that gets in contact that it's "just gone" and hem-and-haw about how the market is very hot at the moment. They can then show you a selection of over-priced, not as attractive properties and try to extract a purchase from a slightly more anxious version of you.

I'm not sure if this is solvable with an app to be honest. And, you will be competing against listings with attractive-looking properties that aren't actually on the market, so on the surface-level won't look as good. Good luck!