How can I return a lost iPhone to a person?

There is no way to find the previous owner if they are unknown to you. I assume that you have encountered Activation Lock which you cannot bypass without the Apple ID and Password of the previous owner. Without that info, you will never be able to use the device. You can recycle it at any Apple Store. Read this link...

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

If it’s activation locked, it’s already been reset / erased in an attempt to use it.

In the process, any hope of the rightful owner tracking it down were lost.

Whoever reset it clearly didn’t intend on returning it the the owner.

I would assume it was stolen and just wanted to get someone else's money for it, since the seller did not release it.

Distance yourself from this and turn it into the police.