I don't think anyone is suggesting that owning these books should be made illegal though. I am most certainly against that. But if eBay doesn't want their marketplace to be associated with transactions of content they deem inappropriate to their brand, I don't see any reason to force them to do so.
If your argument is that eBay and Amazon are so critical to the fabric of the internet that special rules must apply to them, I wholeheartedly disagree.
They aren't critical to the fabric of the Internet, but they are critical to the online retail sector, which is arguably as important today as railways were in 1900. Its weight has only grown during the pandemic which shuttered their real-world competitors.
I don't have a real proposal in mind and like you I do not think Amazon or Ebay are critical to the stability of the internet.
I do think, however, that the current situation is no longer acceptable. I don't know whether as a society we should consider some sort of limit on influence a given company has, but that is actually what is needed. If Ebay was a corner store, I would not give half a shit that it stopped carrying Seuss. When a publicly traded company does it, it quickly becomes problematic.
They're a publically traded monopoly marketplace with special legal protection from being sued. I would be fine with them being forced to act like what they pretend to be- a commons, a market.
The freedom of a giant, monopolistic company is not aligned with the freedom of humans.
We already have precedents for conpanies being forced to impartiality- utilities. Ebay's a utility, the modern large-scale version of the ground a flea market sits on.
If your argument is that eBay and Amazon are so critical to the fabric of the internet that special rules must apply to them, I wholeheartedly disagree.