I am assuming good faith question and I will respond that way.

In US, the entire market is divvied up between various oligopolies. You can name an industry and you can usually find 3-4 dominant companies that drive the market. Ebay and Amazon have ridiculous market power and reach ( reinforced by Covid ), which effectively means that if book is not available on their virtual shelves, that book does not exist for the general population. Ergo, removal from Ebay equals removing of an idea from the 'marketplace of ideas'.

I am oversimplifying, but not by a lot.

It is not outright ban, rather banishment to a very inhospitable place, a Siberia of ideas, so to say. (Russia often sent dissenters to live in Siberia.)

If eBay was a tiny company, it would be nothing, but you cannot ignore the effects of scale and market dominance here.

It is similar to the smartphone world: if Google and Apple block you, in practice you are destroyed.