Selling in the media and celebrity world isn't always for money. It's about using something you have that's considered an asset, that you can trade for positive coverage or other advantages.
The royal family have been selling their cachet and have been a big part of the celebrity world since the 1960s, when Philip decided the only way for the House of Windsor to stay relevant was to play ball with the media.
Their asset is their mystique and their ancestry and the national image. This props up an anachronistic institution. The other side of the coin is their dysfunction, which is worth £££££ to the media. This is the trade off and though they may claim to hate it, it is the price of their privilege.