buying a used phone

There is no way to check if a device is activation locked (Find My iPhone Activation Lock - Apple Support) without having that iPhone physically in your hands. Activation lock has nothing to do with the iPhone’s IMEI.

If you buy a used iPhone or iPad that is activation locked, and do not know or do not have access to the person who knows the AppleID and password used with find my iPhone, you will never be able to activate and use that device. And Apple won’t help you with that - they do not get involved with private party sales or exchanges of used, personal property. You will own a useless hunk of plastic and metal.

There is no way for you to ensure it's not activation locked without putting your hands on it.

I strongly recommend against buying used iPhones from someone you don't know personally. You may very well end up with a brick.

rajivd007 wrote:

I checked the IMEI number, all is well so far!

Hate to break it to you, but that's not possible. No 3rd party has legitimate access to that information at all.

No idea what you mean by "register at Apple to ensure its authenticity"...

I imagine the answer to that is going to be no. You can check the serial number to see if it's under warranty, but all that does is verify that the serial number you entered is valid...