These are Warlock- and Mage-only skills which can net you some money. It's not much, but a few gold is worth it. Note that Mages have an easier time with this than Warlocks as Warlocks need to be at the location for the summoning and need an additional person (a 3rd) for the group to help out with the summons. Mages only need the reagent to open the portal. While the Warlock doesn't incur a cost (except the loss of a soul shard), the Mage will typically be more successful at finding employment using this method.
Buffs. Food/Flasks/Scrolls/Potions/Elixirs are a small, but noticeable, buff to your killing speed and survivability. If you have a max level character that can already supply you with cooking or alchemy, you'll be very well off, because the AH is a cesspool for this. I can't really give you specific foods or potions to look for, since each AH is going to have different supplies. Generally speaking, lower level foods are going to only last 15-20 minutes until you can buy the more powerful versions at 60+. Look for anything that adds versatility or your primary stat, and buy stacks for each level range(1-60/70/80/90/100).
If you can answer all of those questions with yes, then you are in a position to make a lot of gold! You can continue doing every step to get to the gold cap as quickly as possible. The security achieved through this can buy you virtually anything you desire in the game, including achievements that would otherwise be impossible on your own, which leads us to the final gold making tip - Selling achievements. If you are the leader or officer in a guild, you can take good acquaintances on your raids with you for achievements like finishing ICC or any other hard raid achievements that your guild finds easy and manageable. I have seen achievements go for 20k+ if you find the right client! This is very situational, however, and is by no means necessary to hit the gold cap, it just helps!
The majority of you (myself included) don't find farming fun. You don't have 20+ hours to burn each week on an activity that you find dull and mindless. Because of these things alone, you may try but you'll never be able to commit to doing this week in, week out, and this is why you'll fail. Repost after repost, without seeing success, you'll simply call it, and be just another guy who saw a transmog youtuber, tried to get into the transmog business, and quit after a day/week/month.
A little bit about myself for credibility: I am a 21 year old college student and have been playing WoW since it was first released. I have three characters at the gold cap and want to share my methods with you in a way that I hope has been more clear and concise than the other guides out there. Thanks again and check back soon for my role-playing guide for WoW as well as other guides for MMOs and RPGs in general.
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.