Infinite Gold

Method:

Complete Part 1 of "Secret Fish and Where Them", here: Secret Fish of Mechagon - Achievement - World of Warcraft

You will get Secret Fish Goggles that you can use and you'll see little lootable bubbles around the world, as part of Secret Fish and Where To Find Them:
Secret Fish and Where to Find Them - Achievement - World of Warcraft

Go to Kun-Lai Summit, Mount Neverest, with a vendor mount of your choice.
Make sure you have NO OTHER RARE FISH IN YOUR INVENTORY, or you won't be able to loot the relevant bubble.

There is a lootable bubble with 100% drop rate on Thin Air Flounder (Thin Air Flounder - Item - World of Warcraft)

You can loot the Fish, vendor it on your mount for 25G (no need to dismount), and repeat this until you're tired of looting the bubble + vendoring.

There is no respawn time, so spam away.

Probably going to be fixed/bannable, but I don't know.