Playing the Trading Post
This is arguably the fastest way of making big money. However, it requires an attention to detail and a good head for numbers. Also a familiarity with Excel or another tracking tool.
The main tenet of playing the TP is “buy low, sell high.” However, there’s much more to know beyond that. You’ll want to know how quickly something buys/sells, whether or not there’s someone else in that item’s market, and when to move on to the next most profitable thing. Remember, you have to be willing to lose some in order to win some. Don’t go into this thinking that everything you do will be a net profit.
All this said, I am not a TP player myself, and as of this time, have not sought ought any guide writers for this section. As mentioned in the top-level Earning Gold post, playing the trading post is not a widely-explained phenomenon, as secrecy is often paramount to maintaining profits. As such, you’re better off doing your own research, perhaps asking friends who may be more willing to help you.
One of the better resources I have found to learn TP flipping is at blacklionsprofit.com. It not only has advice, but it also links to a number of tools that may be helpful in your endeavors.