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.

"particularly if you don't have knowledge of how the document was acquired."

	▲	estearum 17 minutes ago \| parent [-]
		Correct, and there's no way that the private diary of a still-living daughter of a politician was acquired by any method other than theft. 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.