ICloud Unlock: iPhone 5s?

I bought a locked iPhone 5s and can't get in contact with the previous owner.

Has anyone unlocked an iPhone 5s before? If so, how?

No

If you mean unlock meaning it frees it up from the carrier that it was originally on so you can use it anywhere? No.

If you mean it has a password and you can't take it off? You can always use iTunes to do a factory reset on it to get around the password. This will delete everything that is on the phone, but really, if you bought the phone from someone else, they really should have wiped it before giving it to you.

ICloud Unlocks are scams, don't trust them

You can't. You unfortunately may have been scammed if that's the case.

You bought a stolen phone and it can't be unlocked.