What do you do if you’ve got an activation lock and don’t know the previous owner?

Nothin’.

No onr here can help you, not even Apple.

You have a dead, useless device.

Get your money back, if you can.

Read this extensive user tip/link from one of our regular and experienced, contributing users about used devices and Activation Lock

The All Too Common SAD Reality of Buying … - Apple Community

Return the phone from whomever you bought the phone from and get your money back.

No way to bypass the activation lock without the previous owners information.