There have been some updates to the items on BMAH over the years, and thus a lot of items, especially mounts, are not flagged as available on BMAH in the database. Example: Shackled Ur'zul, Should be a quick update, just has to be done at some point.
Reference list: https://www.wowhead.com/black-market-auction-house-guide#items-sold-on-the-black-market-auction-house