how can I restore to factory settings for an old iphone8, no contact from the old owner

Sorry, but you can't given the information that you have provided.

You need the Apple ID and Password of the person who originally set up the phone. Without that information, the phone is basically useless.

Good plan. There is a very good chance that the phone was stolen.

Never buy a used iPhone.......unless......it looks like this when it is powered on.