If the previous device owner is not known nor reachable, then what do you do?

Note that we are not authorized to remove activation lock from any device. And, the presence of activation lock usually means that Apple support will not be able to help you. That is, under your circumstances, Apple support usually will not remove activation lock unless you can provide proof of ownership that they will consider to be sufficient. In this case, you would need and have your purchase receipt for the device from an authorized Apple seller with the device's serial number on it.* You also would need to present a photo ID. 

All I can suggest is that you try to recovery the needed password (or possibly the purchase price) from the apparently not reachable party you acquired the device from. Recovering the password is the only way it can be made functional for your use. And getting it from the person you acquired it from is the only way to so.

*Rarely, an iDevice is purchased activation locked from an authorized Apple seller.