"Facebook Marketplace"
There's your answer.
iPad serial number can be reprogrammed given low level debug access via factory specialized equipment and permission, or take the flash chip out and swap/reprogram it using a socket programmer.
This is usually done to evade Activation Lock or try to make board level repair/upgrade work. Early attempts may leave some necessary areas not programmed then it will work at that time, but not when Apple decided to use those areas in later versions.