Old and new data from the previous owner on a laptop?

I recently bought a laptop. There was still old data from the pre-determiner. So far so good, I deleted all of them, but now he has informed me that he also sees my saved data and I also see his newly updated data. In principle, we share the same desktop, the same favorites, etc.

Anyone have an idea how I can change that?

Account Settings account linked devices and then remove them from the others

Write on google there you can find more details

Change your Google account…

Reinstall the operating system (which you skilfully kept from us).

You can't advise more. Too much information is simply missing. How and where the previous owner can see your data, unfortunately my glass ball doesn't tell me (and neither do you)

You have a great glass ball. I wrote that it is about the desktop and generally the saved files on the computer. Word documents etc. I use Windows 10

Okay, I can see you are far too smart for me. I wish you a lot of fun with your problem and say goodbye here.