How to unlock an iPhone 5s for resale online?

So I found an iPhone 5s a while ago at a bus station and was wondering what I would have to do to unlock it from its current plan and user for resale on, say, eBay or craigslist.

You should probably return the phone to it's owner. You can figure out how to do this by spending 4 seconds looking through the phone.

You'd have to buy and unlock code and they can get very expensive and it's illegal in the US. Just give it back to the guy and collect a finder's fee. You won't get past the iCloud activation lock on it anyway. An unlock code won't ever fix that. Best luck.