Check iPhone factory information
Apple doesn't provide any carrier unlock status. You need to be vigilant and protect yourself from a buyer by using an escrow agent or other payment arrangement / pre-sale inspection. Furthermore, the original Serial Number and IMEI can be unreliable if there was a service event, error in several databases, etc... It's better to have in writing what you are purchasing and the ability to return the goods if you can't inspect them before you pay.

As for iCloud activation lock, Apple has a nice page to check that before you commit to a potential purchase:


https://support.apple.com/en-us/HT201365


Scroll down to the section titled "How do I check for Activation Lock before purchasing a used device?" and the link for that is:


https://icloud.com/activationlock


You need to contact a cellular carrier to clear the IMEI so I would expect that the carrier you will be paying for service would help you out unless they make their money selling hardware and you are cutting them out of their compensation by buying a used device.