Because there are no stop checks for the safety of the animals.  The animals could be shipped overseas, sold as bait animals, etc.  The person buying the animal may live on the other side of the US and there can be no proper follow-up.  It also lends itself to lowering the standards of breeding and promoting backyard breeders and puppy mills which are in very bad standing.  I would like to think that Ebay is concerned about their public image and would want to be on the side of ethical treatment and rights for animals that only have our voice.