This is quite a tough question for me so it's only a short answer. As I previously mentioned on Day 6 I sell whatever I have in my bags or excess of in my bank. This means that I haven't expanded into every niche of the gold-making market, meaning I'm not really sure what all the niches are.
|My banker hanging out in the Stormwind AH (Dwarven District)
Being a battle pet collector and battler I always check what pets are available on the auction house. I've sold a few pets on the AH but it's safe to say I've bought more than sold. This is in part due to the short time I've actually played the game so I haven't managed to acquire of the world event pets etc. One thing I do enjoy about the battle pet market is that the pets can be found under two sections of the AH interface and I like to compare the prices and find a real bargain. I spoke about this previously and I guess it's a bit weird but it's something I enjoy.