That seems like illegal fraud on the part of Amazon. They sold something they claimed was new and it wasn't. They have to give you a full refund if you return it.

And what is their recourse if Amazon just says "nope, deal with it"?

Sue them? Even assuming that kind of time, money, and energy expenditure is within their resources, Amazon's legal department is likely to be able to stonewall until they run out of money.

Then multiply that by however many thousands or tens of thousands of customers Amazon has done this to. No more than a fraction will ever complain, no more than a fraction of those will ever sue.

Amazon has been allowed to get too big, and the usual methods of dealing with fraud at this level simply don't work reliably.