Activation unlock my 2nd hand purchased device

Return the phone if you bought a used iPhone which is activation locked. The person you bought it from either knows the Apple ID and Password or they sold you a lost or stolen device. You cannot bypass activation lock. Apple will NOT help you. We cannot help you. If the seller cannot or will not help you, you've been scammed.

Read this user tip I wrote about used devices --> The All Too Common SAD Reality of Buying … - Apple Community

Contact the person who sold it to you on Facebook and give them this link: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

If they won't do what it says in the link return the iPhone and get your money back.