"This move works even if the user has no item."
http://www.smogon.com/dp/moves/trick
Are you sure?
http://www.youtube.com/watch?v=r3wOarA1lhk
At 1:44. My Item was removed from Knock Off. I used Trick, and it failed.
"This move works even if the user has no item, but will fail if the user or target has had an item Knocked Off." Read a little more. Don't know, maybe this was written later, but read through once more.