Why is it an issue for eBay to ban it when there's already a list of prohibited categories of items?

It's easy enough to download a copy of it if you're interested in banned items.

Because it's a value judgement. It's one thing to say "we don't deal in porn and guns" but to then pick specific media they do deal in and say "oh and also this one" imparts the kind of fine-grained value judgement that it's inappropriate for an eCommerce platform to be making.

I see your point, but there's also value judgements with other categories. How is an "adult" or "pornography" item defined?

Personally, I don't have an issue with a business choosing to ban or allow items as it's their platform and their business choice. If they choose to not sell any lithium batteries due to their flammable nature, then that would also be acceptable and not in my mind dystopian. There's arguably some risk in being involved with sending potentially offensive media through the mail.

it's a private company

Is there any more dystopian than Google editing Frank Zappa’s track listings because of a single track?