Who is selling these and making a bunch per hour?
I think sellers would do well to notice special things about stamps too, like cancels and postmarks and markings, special labels like airmail, etc, paper of embelopes, names and hand-writing, watermarks of course, perfs, topics, themes, all kinds of special little things that the main 20% of people do not notice or care much about.
But the 80% of people (read collectors) who do notice some of these little inconsequential things will pay more for them as they are special to them.
The economic long tail rule that
Amazon thrives upon, and
ebay too, come to think on it.
Someone always wants something but not right away. Maybe in a week, maybe a month, maybe in two years, but sometime. Selling is looking out for people and looking after them in special ways that others are ignoring.