My sister was trading with a random guy over the PSS, and offered a Cyndaquil. Then the guy kept showing her a shiny crobat with Pokerus. No matter how many times she tried to offer a different pokemon the guy kept showing crobat. So she traded for the crobat. However, when she showed me the Crobat, I noticed it was non-kalos. Naturally I have no way to prove this, but I don't think its legit.
If he kept showing Crobat, he might have been stuck on "Offer". If the other person you're trading with doesn't change Pokemon, you should offer something and then cancel out the trade as a formality.
If it's non-Kalos native, it might be fairly easy to hack check. Head over to the Hack-Check Thread, follow the instructions on the first post, and someone can check it for you.