Although you usually don't want to buy items to sell from vendor, some items can be sold for much more than you pay for them from the vendor. There are a few reasons people will buy a vendor item for a higher cost at the AH. They vendor may be hard to get to, the recipe only sells in limited stock, or the buyer simply may not know where the item is from.

The steps from section two should be utilized and repeated until you reach about 25,000 gold. Once you hit that mark, two more options become available to you, the neutral auction house and cross-realm transferring. The latter is the single most effective way of making gold, although much planning must be done prior to it. However, the smaller methods you used to hit 25,000 should still be done as regularly as before. Nobody got to the cap with only BoE epics, it takes time and patience!

Now, on the other part of the equation, the items that return materials on mission finished are overlooked. Looking at this Wowhead comment the most lucrative ones are the disenchanting rod (returns enchanting mats, best case scenario, 10x veiled crystals) and the magnetic mining pick (returns ores, up to 40x Stormsilver). This is totally server dependant, but you will need to check which ones are worth where you play.
The cloak is nice because you only need one of these instead of the previous cloak versions since the primary stat changes depending on the class it is used on. The on use trinkets for versatility, haste, or critical strike could be very strong if used together by numerous party members with the same trinket. Finally, the threat reducing trinket could be useful for solo levelers in dungeons with newer, non-heirloom geared tanks in order to not pull threat on some larger packs of enemies.
First of all, the value of transmog is decided by rarity and how they look. Some items like Minenarbeithut der Tiefen or Pendel der Verdammnis (they can drop in Uldaman) are worth millions of gold. They are worth so much, because they have a really small chance to drop. Other items are bought well because they look good. Best example in this case is the Glorious set, its legplates Glorreichen Beinplatte reach a 100.000 Gold value easily.