Is my used iPhone 6s a fake one, can apple deactivate it?

I recently bought a used iPhone 6s, when I enter the serial number to coverage.appple,com, it states “We’re sorry, but this is a serial number for a product that has been replaced.” There was originally iOS 10.3.3 and I was able to update it to iOS 11.4, I also reset the phone and it re-activated normally, I was also able to use my apple id and restored from iCloud backup. I consulted with a local tech service, this receptionist lady said she can tell if its original or not, and in her opinion the screen and body are not original, I yet don’t understand whats the case with this phone.

After my previous iPhone broke, I was able to purchase this one after some time and difficulty, I am worried of thinking that i got scammed.

If its a scam and considering the phone is already activated, is it possible I can use this phone or Apple can block it to be unusable?

Thanks in advance!