From what I see, there are quite a fair bit of sellers that loves to use "Unfilled warranty card". Technically, this is stating the actual fact but it may be our perception (or the buyers) that this would mean that we can still register for the warranty. If the seller said "Unfilled warranty card that you still can use to register for warranty", when it cannot be, then it would be a false advertising or in some cases a scam.
Likewise, as a buyer, I faced the risk if I choose to buy an item without the original receipt to see the date of purchase / unfilled warranty card. Right or wrong, its up to each individual opinion but to me, scamming would be when the seller states that the warranty is valid (i.e., you can still use the unfilled warranty card) but in actual fact it is not.