This to me is what's most likely happening here
might be the dude buying who is also the same dude trying to scam.
it doesnt make sense to me that someone else would know the trade happening right there and then, down to the right item type.
I thought the same thing until it happened to a friend and someone I've been trading with for a long time. Honestly, I really don't know how they know the trade will happen, but just the fact that the possibility exists is reason enough to be careful.