Whether or not they are out there, it's best not to get involved with those type. Even if it's easy money or promising to give you a high return, don't get in quickly. DYOR and be sure what you are going to and risking. Nothing will ever come out right with a scam.
That's for sure, it's nothing more than an ordinary theft. I bet there are easier and cheaper ways for becoming a criminal.
But let's think logically for a moment, why would any one be selling a method that can bring millions and risk for it to be discovered and patched up? Doesn't make any sense, such skim would be kept a secret for as long as possible, in order to bring hassle free profits.
Fact that it's being sold somewhere, means its probably already patched off the atm's or it never worked in the first place, what is in fact the most probable scenario. Desperate search for ways to rob people, will end up with the seeker being scammed...