https://www.deslegte.com/boeken/tweedehands/

https://boekenbalie.nl/

https://www.boekwinkeltjes.nl/

Thanks for these! One great site is iberlibro. That is the Spanish domain. There are other versions like Abe books. They connect to many bookshops around the world but you can easily make sure you're buying from the EU when you purchase. You can clearly see the bookshop and the country they're in. Really cheap and really good service.

I actually found medimops.de to be quite good as well.