This is a real worker that you can slide right into your act.
A card is selected (say 5D) from a regular red deck, it is signed an placed into the center of the deck, they actually see the 5D going into the deck, no control etc. You remove a blue backed card from your pocket it has a large shark stuck on it. You claim that this is your card shark and he will find their selected card.
The shark instantly vanishes from the blue card, the deck is spread and right in the middle there is one BLUE backed card with a shark on it, this really is their card no switch etc. Now for the killer ending, they slide out the signed card it has a LARGE BITE OUT OF ONE CORNER. This really gets an unbelievable reaction, the spectators eyes nearly pop out. Resets in 2 seconds, a different card each time.