It is unambiguously illegal to pay for goods you know to be stolen. In all 50 states and federally.
Most courts would assume someone who purchased a private diary of a living person would know that it was stolen.
Most courts would assume someone who purchased a private diary of a living person would know that it was stolen.
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.