What to do to reuse lost iphone 5s?

Okay so i bought an iphone from someone i know. He said he got it from a guy, but the phone is most likely stolen or lost. Besides factory restoring this iphone what else do i need to do to be able to reuse it?

Likely wouldn't be able to anyway because of Activation Lock which would require the previous owner's Apple ID.

Factory restoring won't help you remove the phone from blacklist. You need to get the phone unblocked before you can use the phone again. If you need help with getting the phone unblocked, mail me for references.