The heading explicitly excludes unused stamps that are current and valid for postage in the country of destination (i.e., the US). An unused foreign stamp is generally treated as a collector's item because it cannot be used as valid US postage. The classification issue arises mainly with US stamps being re-imported, where CBP would look at whether the stamps are current legal tender for postage or are being traded purely as collectibles.

Yes. Stamps, covers, and philatelic lots originating from or depicting imagery of OFAC-sanctioned countries (such as North Korea or Iran) may be subject to import restrictions even as collector's items. OFAC has issued general licenses covering certain informational materials, but the scope is fact-specific. Importers of philatelic material with a sanctioned-country nexus should consult with a trade compliance attorney before importation.