They don't. They sell them via Amazon Warehouse as second hand, often in perfect condition but with opened packaging. Sometimes minor scratches etc. It can be a great way to buy some things.
If there's something missing or noticeable they'll tell you in the listing.
I've learnt from experience to be very wary of 3rd party sales on Amazon. So much cheap crap. Never understood why Amazon cheapen their site with much of it!
It's ruined the quality of the Amazon brand to me. I used to buy a lot more on Amazon than I do now. There's so much junk on there, a lot of it of such poor quality that it should never be sold at all, and then there's also a lot of counterfeits. God help you if you ever want to buy, say, a replacement phone or laptop battery, or charger.
I buy a lot on Amazon and I had a very odd experience the other day for the first time. I ordered a board game expansion and when I received it there was a sticker from Amazon Warehouse deals on it. I certainly didn't order that and the item was slightly damaged so I replaced it with a new item instead. But it was the first time they ever sent a AWD item instead of a new item.
If there's something missing or noticeable they'll tell you in the listing.