howto find previous owner of an iphone

That's how it works. The previous owner locked it, either failed to unlock it before selling it or had it stolen, and then it locks up when you update it. YOU ARE NOT GOING TO BE ABLE TO USE THE PHONE. THERE IS NO WAY AT ALL OF YOUR BYPASSING THE LOCK. Only the ID and password used to lock it can unlock it.

Does your screen look something like this (with the ID hint being not yours)? -

If a used iOS device has been 'Activation locked' then only the previous owner can unlock it, either by providing you with the account ID and password, or by removing it from his list of devices (as he should have done before selling it).

If you are unable to contact him to do this then I'm afraid you will not be able to use the device - there is no other way of unlocking it at all. Apple cannot unlock it for you, nor find the previous owner for you, and neither can the websites that offer to unlock it for a fee: these are scams.

There is no way to find the previous owner of an iPhone. And it probably wouldn’t be a good idea, anyway, because if Find My was enabled when you bought it the phone was most likely stolen.

If whoever you got it from can't help you, there is no help to be had.

Turn it over to the police.

You bought a stolen device.

The shop has sold you a useless phone. Take it back and demand a refund. Whether or not it was originally stolen is academic: you are never going to be able to use it.