I haven't seen anyone selling new OEM DJI parts (other than the link you posted above). Most people are disassembling new/used Mavics and selling the parts (like Thunder-drones is doing).
Yes it is very strange and I also find it strange that none of the visible UPC numbers on his packages are in the UPC database. If they are used parts he certainly went to a lot of trouble to make them look like OEM parts.
Rob