[estaseuropano]

Amazon does not separate the inventory so you will never know whether what you get is amazon or third party inventory.

[alicesreflexion]

This depends on the item. Items that just use a UPC do get comingled, products that use an Amazon-specific barcode like an X00 do not, and this is required for some types of products (eg food or anything with an expiration date)

As I understand it, anything used or returned will also get an LPN barcode, which tracks that specific unit.