@Timm,
your link is to a page with normal stamps. The secondary post listing is this:
https://www.stampi.nl/EN/M/Personal...115/page.phpThese are personalised stamps commissioned by private individuals or companies. I never heard of "secondary stamps" but this terminology appears to refer to these being offered in a secondary market, i.e., ordered from PostNL and then sold on.
PostNL also issues similar themed sheets of stamps. The catalogue lists these as personalised stamps. The latter are official issues by PostNL.
The official issues are:
https://www.stampi.nl/EN/M/Personal/2003/page.php