Why on Earth not? If you can tell it is real, that little piece of paper isn't going to make the knife any better.
I have bought several here and on fleabay, some with papers, some without. All real. All didn't seem any worse from lacking that useless little slip of paper (or the box).
All were used of course. A new one? No. A new one should be new, never used, all things included, not sharpened, not carried, not handled, not scuffed, not rusted, etc. I DID trade into a "BNIB" Umnumzaan that had the lanyard removed, a denim string stuck under the clip, lint in the handles, and it looked as if a chimpanzee had used a Sharpmaker to knock the very tip off of it. That one DID have the birthday card....