Commingling is certainly something I wasn't aware of, it looks as if most consumers are in the same boat.

The idea that FBA doesn't actually get me parts from the seller I picked has never crossed my mind. I certainly wouldn't have bought quite a few things if I knew that possibility existed.

It's part of their distribution strategy. I'd generally send 500 units to their warehouses a week. When you're dealing with quantities that large they have you split it up to several shipments to different warehouses throughout the country.

Usually it would be many small quantities near me, and then one large one to the other coast which they would then distribute amongst their warehouses on that coast themselves.

So as the number of vendors stocking inventory increases, the cost to amazon to distribute inventory widely enough to hit SLAs goes down because more of the cost is borne by the vendors.

I guess I see how this can happen for everything marked as "New" with no extra details, but how does this work when the ones from different vendors are in a different condition (missing box or whatever)? Are they still commingled then? And if so then how do they make sure the condition matches the one they send you (e.g. not give a missing-box as one that's factory-sealed)?

Used items are not commingled with new items.

But what about with other used items? It's the same problem either way.

And what about "New" items missing packaging etc.?

This isn't an actual problem Amazon has, and Amazon has enough problems that we don't need to make up new ones to discuss. Used items are tracked and sold individually (with their unique condition described and viewable by the purchaser), unlike new items. They're not just binned together by condition, when you buy used you're buying a specific item out of inventory. A "new item missing packaging" would not be sold as a new item. "New" items that are returned to Amazon are re-sold as used under the Amazon Warehouse Deals brand, even if the packaging looks factory sealed, for example.

Thanks!