Don't be fooled into thinking Amazon and eBay are doing this because of any moral imperative. It ultimately boils down to the almighty dollar, and these large companies are so scared of being on the wrong "side" of this that they trample on the rights of individuals. What happens when other marketplaces prohibit these books? What happens when there is no legal outlet to sell them outside of face-to-face transactions?
Mark my words, one day the left will lose their position of "authority" on these topics, and the same tools will be abused against the very people who sought to implement them. It will be a sad and unfortunate day when that occurs.