icloud locked iphone

On the outside possibility that this was simple ignorance on the part of a seller disposing of their own personal device …

… contact them and ask that they perform the following actions:

How to remove Activation Lock - Apple Support

The fact that it's activation locked is almost a certain sign you purchased a stolen phone. Apple will not help you, so there's no point contacting them.

If at all possible, return it for a refund. Otherwise, you will forever be stuck with a useless brick.