"particularly if you don't have knowledge of how the document was acquired."
Prosecutors don't need to prove the buyer actually dispositively knew the document was stolen, only that reasonable person would have known it to be such.
Which would be obvious in this case.
But I appreciate you iterating on this -- I understand your position and while I disagree with you on the question of what "reasonable" would be in this case, I absolutely think that if I could read minds I would find that Project Veritas staffers at the very least knew the diary was stolen.