I've fired up the ol' blog here in hopes of squashing some questions about keeping your World of Warcraft account active with in game gold - Or instead amassing a dragon's hoard worth of gold with minimal effort and time invested.
So to get into it, there will be a few things you'll have to do as prerequisites, which will mostly fall on the shoulders of collecting and leveling some key battlepets to 25. If you're an avid collector already this may not be too big of a deal, but if you're just starting out this can be a huge undertaking. I would also strongly recommend the addon called PetBattle Teams to keep things organized which you can find through the Curse client here. So lets get into it starting with some battle pet dailies you can do for a chance at some serious dough with very minimal effort once you're set up.
Panda Spirit Pets
So, in order to farm these pets you first need to complete a quest started by Sara Finkleswitch if you're alliance, or Gentle San if you're horde - both of which are standing on their respective Panda hub outside decks. More information about this quest found here. I would recommend upon completion of this quest that you select the Pandaren Water Spirit as your reward, as it is the most useful of the spirits to add to your arsenal. You can find easy strats to defeating each of the spirits here.
Once you complete the quest, you can battle each of the spirits daily once per account for a chance at getting the respective spirit you're battling as a pet. These pets range anywhere from 1k to 10k depending on the pet and your server.
My method to farm this is simple. Roll fresh characters on an empty server and have a friend (or any other helpful player) fly one to each of the Pandaren Spirits and park there indefinitely. Each day you just log in your characters one by one, completing each spirit's battle, and moving onto the next one until they're all finished. If you don't feel like doing each spirit, at the very least I would advise parking one character on the Pandaren Water Spirit to fight daily. This pet is the most expensive of the set and therefor is worth the most gold for your time.
While Darkmoon is in town there are quite a few lucrative ventures to capitalize on. Farming Moon Moon, The Darkmoon Rabbit, Fishing for Darkmoon Daggermaw. or simply doing the repeatable daily quests on all your characters can net you a fair sum. I'm not going to go into any of that since sitting on spawns or grinding through the mini-game dailies can get old very quickly.
As stated above with the Pandaren Spirits, I would advise making another character exclusively for when the Darkmoon Faire is in town. Park this character next to the Darkmoon Feasel Pet Battle Dailies and fight each of them for a chance to get Syd the Squid as a battle pet each day.
Ahn'qiraj 40 Raid
This one isn't a pet battle but is instead an old raid I run for the explicit purpose of obtaining some very sought after pets. The main focus of which being the Anubisath Idol. However, there is a boss that is skippable within the raid that most people don't tend to bother killing named Viscidus because of it's particular fight mechanics.
Viscidus can only be killed by first freezing him with frost damage, and bursting his frozen block of ice with enough hits per freeze time to shatter the ice.In my experience only a few class/specs can easily handle this and they are as follows: Beastmaster Hunter, Windwalker Monk, Frost or Unholy DK, Outlaw Rogue, Fury Warrior, Enhancement Shaman, and Havoc Demon Hunter. I will post another entry about this fight since it's so niche. Because of this however, people tend to just skip it altogether. Meaning theres money people are missing out on and less contest for this pet's market.
I know, I know, you never go to your garrison any more. Hear me out though -t he pet battle menagerie has some really good rewards still! You'll be rewarded with battle pet tokens that you can spend in Legion-Dalaran at the pet supply shop - including available pets and buff items, the former of which can be sold for profit and the latter of which can be used for massive battlepet gains if you're in need. Also the menagerie rewards you can contain straight up pet leveling tokens to throw onto some scrub pet you're pitifully trying to level up here and there - or just boosting a pet you want to sell at level 25 (Like the Anubisath Idol mentioned earlier). You also have a chance at getting a random set of pets as loot upon completion! There's more of a demand since not as many people are doing these in Legion content, which makes for a win all around.
I do neglect the menagerie fights more often than not, but sometimes I check in to see what the rewards are. If a fight is quick I'll run all of my characters through it just to get some fast and easy gains.