I thought that ebay was just a website where people could sell their historical documents to each other. Ebay does not "disseminate" the things that people send to each other. Heck, it doesn't even deal with the delivery! It just processes the payments. Why are they willing to engage in editorializing stuff that they don't even see? It makes no sense.

Ebay is a company, not a website, and company policies specify which products can be listed by their users. It's not neutral, you're being deliberately obtuse here.

But it should be, given it's a monopoly on the space.

It isn't a monopoly in the space. There are so many widely used marketplace products at this point.