On the GTS, people will hack stuff and ask for things that NOBODY has. Once, I saw a clearly hacked shiny Lucario on there and they were asking for a level 100 Cascoon! I think they need to get a grip!
they are probley just cloning and if you don't want to lose your good pokemon then you look for somthing no one has and how would you know the Lucario is hacked?
It's happened before and there's absolutely nothing we can do about it. The worst that can happen to the person is a trade actually commence. But as Shadowmlb said, they could have been cloning. I was trying to clone my Saikyou Milotic and I ended up getting a trade. Who knew anyone had a Level 100 Abra? But now, I ask for Shaymin and Darkrai when I clone.