Although WoW currency can be found through quests, fallen enemies or the dreaded "farming" process, doing it the conventional way is time-consuming. Considering the upcoming release of the Legion expansion pack in August of 2016, stocking up on gold is more important than ever. This way, you can spend less time farming and more time buying everything you need to tackle the next set of challenges.
Set sail for the previously unmapped isles of Azeroth. Battle in groups of three as you race against cunning rival intruders—or enemy players—to collect the island’s resources. Constantly evolving challenges await as you traverse frozen landscapes near Northrend, open the mysterious gates of an abandoned Gilnean castle, navigate a war between elementals and more.
2. Island Expeditions. What?! you may ask, but it is simple. The transmog that drops, now rather frequently, from the expeditions sells well and for high amounts. Despite its somewhat common drop rate the fact that the loot table is so large means that people seeking certain transmog will be more likely to buy it than farm it. The parroted meme on r/wow that it is unrewarding only makes it better for someone like me. In the past two days from doing Island Expeditions on the side (about 20 total) I got 6 transmog items of TSM values between 15k and 150k. I sold 3 so far totaling 140k (Plundered Squid Caster: 50k because I wanted to be rid of it, Stinkrot Smasher: 69k, Razorfin Breastplate: 28k). Pet drops are a different story and while you can sell them, the fact that they need to be looted for the achievement means that the potential buyer pool is smaller.
I'd like to add my personal experience as of a week ago in regards to illusions: I bought all of them for 10 silver being the highest bidder. They sell for 2-4k each usually, but it's a very slow market with little interest. They might be a nice addition to your portfolio if you can get the mats with little effort, otherwise it's not enough to warrant investing in it. At least on my server.
There's a few profession crafted pets in BfA on the horizon - Battle for Azeroth Tailoring is getting a Hot Air Balloon style pet, and BfA Alchemy looks to be getting a brand new Transmute Meat to Pet. There are some mount buff consumables from both BfA Leatherworking in the form of a mount barding, and Blacksmithing in Battle for Azeroth will be able to craft Stirrups and Hoofplates to increase mount speed and allow you to interact with things while mounted.