The big chains advertise things they only have one of, I was 3rd in the queue at PC world a few years ago for a graphics card, it was a £300 card for £99
I ran to the graphics cards, looked and there were none so I asked the sales guy, his immediate response was "we've sold out"
I pointed out I was 3rd in the door and there were none on display , I pushed the point and reluctantly he said i'll have a look out the back where amazingly he found one.
for the most part it's a con, if they are "giving them away" there'll only be one and you'll be really lucky if you get it