Possible counterfeit / copy product or Amazon drop shipping scam?

I've created a book which I own the trademark for, I get manufactured in the UK and I'm the only authorised seller (I don't offer it to distributors). I sell on my own website and sell on Amazon FBA.

I've noticed sellers on Ebay, selling it for less than the RRP. That led me to believe it was a copy or counterfeit, perhaps out the back door of the manufacturer or something. I've ordered a copy from one of the Ebay sellers to check it out. It has arrived the next day from Amazon (Amazon packaging etc).

It has my barcode / ISBN on and seems to be one of the actual copies of the book. It seems to be a drop shipper (there are multiple on Ebay offering the book).

My question is, how are they selling the book on ebay for £14.99, ordering it from Amazon for £16.99 and making money? (Ebay price £14.99, Amazon price £16.99)

Is there some kind of discount they're getting? I obviously can't see my order through FBA as customer delivery addresses aren't shown but it's very strange.

Can anyone shed any light?

Thanks in advance.