What a little bastard the prankster is, although it wasn't the buyer's fault you'd think he would check with OP first for such a great "deal".
You should have been watching. If someone picks your stuff up and walks away with it then YDI. If they try to argue the price tag, tell them you didn't put it there. If they still argue, tell them it's not for sale. Even free is a price, and you can refuse someone's offer to take it.
Better question is, how would you know if the tv had a "free" label on your tv without you seeing it? Because if you saw it im sure you would have taken it off.
I would've kept something so valuable near me to ensure its safety.. or if you walked around your things I would've checked for that sign or someone taking a large object..