"We're sorry, but this is a serial number for a product that has been replaced. Please check your information and re-enter your serial number. If your information is correct, you may need to Contact Apple"
So I called Apple, and they said that when they look up my device it shows that the device is out of warranty. They said it should be fine, just maybe it was refurbished or had work done by a third party. I wouldn't know if I could get support for it until I contact an Apple genius when I need some hardware repair. I am eligible for phone support they said though. Nonethless, it does not show that the device was reported stolen or anything. I don't know if this is normal for second hand devices as I've always had my phones through a contract or directly from the store. Albeit the last device I bought new was 5 years ago so things may have changed. When I looked up the serial it would show when it was purchased and that the warranty for support has expired.
My current bought used device doesn't show anything when running the serial number except for that warning message.
Anyways I am going to try going ahead and using this device and see how it goes. I don't know if I were to buy another device if it will show the same message on Apple's website when I run the IMEI either.
Your own experience may vary though. I am just sharing my own exposure to this type of thing if that helps.