Apple ID halfway merged accounts with Unknown person

i bought a laptop off someone and they didn’t remove their Apple ID. We met up and tried to reset it. And reset it. But we somehow ended up merging accounts in the process. I have all his passwords and everything in keychains. So I assume he also has all of mine. We don’t know each other and I feel like our accounts are no compromised. No guarantee that we would manually delete everything of each others as we are not in contact.