Most of the Amazon X products I can think of are really basic obvious things. Sheets, cables, simple computer peripherals... well OK those are the things I remember ordering offhand that were such things.

My point is that they'd be categorized as basic staple items. Someone else might have sold a particular size or feature of X, but I don't see any reason why Amazon or another competitor wouldn't think to copy it.

Instead (as I made in a different reply a couple hours before yours) I believe it's the strength of Brand Recognition as a form of Quality Control, which is an area that has decayed on Amazon as a whole but less so with their own brand of products.