Rule #1 when playing on the Internet -- don't believe everything you read. Until this is officially listed in Amazon's return policy, I think it's fair to say this is nothing more than hearsay.
I'm not trying to win. Just simply trying to understand why you created this thread. And, I'm also sharing facts so anyone else reading along knows how Amazon really works (rather than how you've heard it works). Drumming up a bunch of rumors is not helpful for anyone.
One last time, I will try to put this simply. I feel that the different types of Prime, makes it difficult to be sure what you are ordering and from whom. Now, you have not gone out and searched Amazon for the 3 types of Prime that I have mentioned it would seem, though I showed one. Agreed, it that case the seller was also the one that fulfilled the order or sent it. But, it is my belief that this order, when I made it, stated Fulfilled by Amazon, but sold by XXXXXXXX. But I cannot prove it, I admit that.
So, why would I come and lie about this? I have nothing to gain by it. So, if you listen, my intent was to let people know that they might think they have rights or perhaps better, options that they actually do not have.
I understand that you are a vendor. You perhaps have never had this happen, that too I understand. But it did happen to me, I am trying to help others, so they are completely aware of what Prime can mean or what it does mean as far as returns are concerned.
Did you not read the other poster, that stated he had seen Prime orders that were fulfilled by others, due to lack of stock?
Please, there is nothing more to talk about. I have tried to explain the WHY as best I can, if this has not been enough, well sorry.
But either way, please, let's just drop this, I am not wanting to talk about it anymore. SO, not meaning to be unfriendly, I will not answer any more questions. If I haven't made clear by now, I wont in the future I am sure.

If there is anything that I have said concerning this, I assure you that what I have said is what was said to me by Amazon. If they informed me incorrectly, that would not surprise me.