As mentioned it was a replacement unit provided by Apple.
Replacements happen when a device is sent in for repair and cannot be repaired for any reason, then a replacement is issued to the person that sent in the device for repair.
This unit is then labeled as a replacement unit and gets a model number identifying it as such.
Apple does not sell replacements. They are sent back to the owner of the device that was replaced, and they are free to sell it themselves.
There is nothing wrong with it being a replacement. It means Apple did its job and provided a new or refurbished device for one that stopped working under their warranty or repair service program.
Said devices carry the remaining warranty of the device they replace, or 90 days whichever is longer.
It's perfectly fine that someone sold it. It's a working device.
Its activation date is not really relevant. It was activated when the user that received the replacement activated it.