1. You don't save credit card numbers...in fact you don't transmit anything from the form...just clear it out without saving.
2. You don't tell them "sorry we are fake"...you tell them "This product is no longer available for sale...your credit card will not be charged".
3. You have to remember this is a limited run. At most you'll have 10-15 buyers before you realize that you are onto something and would stop your test. So the chance of someone bitching at you for "taking" their credit card number is slim.
So there is no fraud, no identity theft...the only unethical thing you are doing...is making someone read a description of a product that's not available for sale.
And in return you get your REAL numbers...not the inflated ones that can send you chasing the wrong idea. I'll give you an actual example...156(order form impressoins) = 18(order form submits) = 13(actual sales)