My understanding is, if I sell an item with a SKU[1] 123456, and use "fulfillment by Amazon" it goes in an Amazon warehouse in a bucket with all the other items of the same SKU using Amazon's shipping. Then my inventory becomes just a number in a database.

People selling DVDs on Amazon were complaining about bad reviews because someone bought from them, but got a counterfeit sold by someone else.

1: https://en.m.wikipedia.org/wiki/Stock_keeping_unit