How can I trust in apps signed by you? How I can verify that's the same signature? Your app not even (yet) has integration with AppVerifier like Accrescent
Discussion
Inspect the source code and build the APK yourself. There is a file integrity hash check and an APK certificate hash check but Android enforces this validation anyway.
For first installs you're choosing to trust AppVerifier and not zap.store, that's okay. I can't change who you trust.
However, developers will start signing apps via nostr events so on zap.store you'll be able to check that with your web of trust (via a service or manually)