People will buy something from a store, buy a fake version of that thing on AliExpress, swap the fake into the box, re-shrinkwrap the box, return the shrinkwrapped fake to the store, and sell the real one on eBay.

The store will often put the thing back on the shelf, because it's still shrinkwrapped, weighs the same, and even looks the same under an x-ray.

And then someone buys it and gets a dud.

This is why, except for a few high-margin categories, Amazon itself will never put returned stock back into inventory, instead always selling it off through bulk channels. It's effectively impossible to authenticate a return as legitimate these days, without converting the "BNIB" device into a "NIB" device.