He scammed you. He wanted a new phone but still had money left to pay on this 13 PM he had with Verizon and didn’t want to wait (or wasn’t willing) to pay it off.

So, it’s not as bad as being sold a stolen device but it’s pretty useless to you - the scammer got what he wanted - money for a phone he no longer cares about so he can go elsewhere.

Always meet face to face with these kinds of transactions - meet in a public place like a police station (many even have areas set up for online transactions) and ask the seller to wipe the phone so you can activate it. If he’s unwilling/unable to do that, something is up.

Once the phone is in your iCloud account, give him the money with confidence. If he puts up an argument, just say he can have his phone back and he can reactivate with his account and restore.

A legit seller should have no issues with this,