unlock iPhone

You can't if the previous owner is not helping.

Activation Lock is a theft deterrent feature with no bypass or workaround.

Without the owner's authorisation (Apple ID & password), no thief or anyone including you can re-use the device.

You took the risk and lost the money, so lesson learned?

You didn't buy the iPhone from Apple so what do you expect with stolen goods?

Same if you bought a stolen car, does the manufacturer have something for that?