Yup. Multiple sellers can sell the “same” item.

I put same in quotes, because it can be counterfeit or just similar. You’ll never know if you just buy the cheapest via buy now.

The only exception if it’s your own brand. Which is what many sellers are doing now. That’s why you are seeing a billion of poorly branded, cheap Chinese products.

Yes, that's the root cause of many of their fulfillment issues. And it makes sense for something like a mass-market book. It makes less sense for other things.

Yes, they do. It's one of the reasons you can buy a "new" hard drive that already has several years use on it.