I don't see how anyone could make mistakes like that. You can tell it is VERY skinny, probable URI, cloacal injury and the weight isn't anywhere near what the seller claimed it to be. I'd understand if it was 1,600g, that is fair enough, but there is a 500g discrepancy here. That is almost 1/3 of its weight missing! That is a MAJOR difference! I'd love to hear the sellers side of this story.
I'm a nice guy too, but I would throw a fit! I would never conduct such shady business. It sounds to me like this seller is playing you for a fool.