Because it isn't transparent to the purchaser that the stock comes from an illegitimate seller. So as a hypothetical example: Rolex makes Subarmariner watches

Because it isn't transparent to the purchaser that the stock comes from an illegitimate seller. So as a hypothetical example: Rolex makes Subarmariner watches and sells them on Amazon, and KnockOffWatchCo makes counterfeit Submariners and sells them as genuine ones on Amazon. I order a Submariner watch from Rolex through Amazon. However when Amazon sends it to me it grabs a watch from a bucket that contains intermixed submariners supplied by Rolex and counterfeit submariners supplied by KnockOffWatchCo, even though I purchased it from the legitimate seller. This is because Amazon treats items with the same SKU regardless of the origin of those items. This leads to two problems: the first is legitimate sellers being blamed for selling counterfeit goods. The other is the damage to brand perception. "Oh wow my Rolex watch sure keeps terrible time, I guess they've gone down hill". So in the case of eBay if the seller sold me a fake product and I could identify that it is fake I could dispute that I received the advertised goods. With Amazon I could do that too, but I would not buy through that seller again even though they weren't actually the ones who provided the item. Does that make more sense?