I used to shop on Amazon, but now I'm all eBay too. "It just works"and even though I know it's another mega-sized company, at least it's not amazon.
More than once though, I bought something on ebay, only to have it delivered in an Amazon box (because they fulfill for so many companies). Wish I could think of a way to avoid that.
It is against eBay policy for sellers to ship directly from Amazon. Report them and eBay will, after several months, end the offending account. Include pictures of the Amazon packaging. If you want to go the extra mile, go to the vendor's storefront and search for their items on Amazon. If you find a bunch, tell eBay that the vendor's storefront is full of relisted Amazon items.
You can also leave negative feedback stating that the vendor simply ships from Amazon at a marked up price.
It is also against Amazon's policy for accounts to use Prime shipping perks to sell items. If your package has a gift receipt, the vendor is violating that policy. The receipt will include their account name, and you can report them to Amazon too. If it doesn't have a gift receipt, they're not violating any Amazon policy, and there's nothing you can do on that end.