A forgery or how to explain?
The listing expired. I do not know if the listing has been changed. But the text next to it reads "4 old stamps, Queen Victoria, One Penny. The Black Penny is a forgery, has no watermark"
Of course, if you want to be scammed, Marktplaats is the place to go. A small chance someone is selling an unknown Rembrandt for a child's painting, big chance of being sold a child's painting for an unknown Rembrandt.