If there is no face value, it is a map, not a stamp.
If there is a face value, but it is not on the one with the Harry Potter pattern, it is probably a "personalized stamp". Print patterns at will with the ticket, and it's no problem to print your head.
Harry Potter stamps have been issued in France, but never in China or Taiwan Province province. The stamps in Taiwan Province are marked Chunghwa Post.
I haven't seen the name China Philatelic Co., Ltd., and judging from your description, it should be just a picture, not a stamp, which is of little value.