There is a significant and fundamental difference between the publisher of the Seuss books phasing out certain volumes (putting aside the reasons good or bad), and the prohibition of trading or collecting those works between private individuals. Yes Ebay is a private company and can do whatever they like, ban whatever items they want for whatever reasons they want, but as a society we should hope they'd have very good reason to want to interfere with what private individuals collect. All kinds of is traded on eBay, you can make subjective arguments all day long about a very wide variety of items for sale on their platform.
The argument that buying or selling copies of the Seuss books in question is perpetuating racism or bigotry itself is incredibly thin. The argument that people shouldn't be allowed to trade those books between themselves is completely unsupportable.
Yes, you can buy the books elsewhere for now. But that is beside the point.