This being my first post, I don't have much experience, so flame all you want, but try not to be too harsh.
First off, I'd recommend you have a level 25-ish character so you can quickly kill the mobs mentioned in this guide. Now then off to the money making...
My recommendation for lower levels is to go to the Barrens and kill the harpies located in the Northwestern area. These harpies have some of the highest drop rates of Light Feathers and are remarkably easy to kill. If you have a class with a strong AoE, you can easily kill 5 mobs every 30 seconds (at level 25). Typically, 1 Light feather will drop every 4-5 kills, though you may get luckier.
On my server (Misha) 1 stack of Light Feathers (20) sells for 15-20g. Within 1 hour of farming with my level 26 mage, I was able to get 4 stacks of light feathers. This means that at level 26, I easily earned 60g! Granted, the prices you see may vary, though Light Feathers are typically in demand by many people and as such, prices are generally high.
My recommendation to anyone planning on using this guide is that you get a high level player in your guild (or just someone you know) to help you quickly kill the harpies, or to simply use a character of yours that can easily dispatch these mobs.
I hope you can earn a lot of money with this guide...
With regards,
Chris W. (A.K.A. Weaverbeaver)