It is difficult to say for sure wether it is a hack or not. A quick google search of the OT gave me this
German site. There it is a shiny pentgon Bagon with the same OT and TID. Would be nice if someone who knows german could translate it, since I don't understand any of it. If that Bagon is hacked, this one is a hack. If the Bagon seems fine, then this Talonflame could be fine. But if you trade it and someone manages to get solid evidence that this is a hack, you will be in trouble. Finding proof that this Talonflame is legit is almost impossible, but that German link is the only way I could see that could be of any help. If the person listing the Pokemon there knows the person that bred the Bagon, you have a hope. Otherwise, you won't really get anywhere with that.
Edit: I think it says there that the Bisharp is a hack, so proving the Bagon is legit is honestly almost impossible. If you really want that Talonflame in your thread, you will need evidence. This link is the only thing that could give you something, so good luck proving it to be legit.
Edit 2: A policy where you only can trade stuff obtained from people on the site sounds smart. Is there any chance this could happen?