Plains of Ashford is by far the best zone I have come across for players below level 20 to make some really good cash. The reason I have found this zone to be so much better than other zones is mostly because of the chance of finding Chilli Peppers here. Chilli Peppers are the best selling and most profitable item I have found in this level range. They sell extremely fast and for usually 1 - 2s. You'll also find the standard lower level gatherable stuff here such as Copper Ore, Green Wood Logs, Black Peppercorns and the like.
In Plains of Ashford you will want to stop by every node you come across whether it be a mine, tree or plant. They're all worth something on the Auction House and you won't make money by leaving valuable materials behind! The biggest money maker in this zone will be the gatherable Herb Seedlings plants (as seen in the screenshot below). These have some of the most valuable herbs in them such as Chilli Peppers and Black Peppercorns. These plants can have other goodies in them as well, not just the aforementioned items.
If you're looking at this guide you're probably still really low in level and you're not fully accustomed to how gathering works in Guild Wars 2. Of course you probably already know you'll need a Logging Axe/Mining Pick/Sickle to gather anything but what you might not know is every single node location changes when the servers go down or are restarted. This means there really are no good set in stone gathering paths in Guild Wars 2. This may seem annoying at first but if you think about it, it keeps some things new and interesting.
The point I am trying to make is I can't provide you with an extremely accurate route you can follow for maximum profit here. That's all on you. However since in Guild Wars 2 everyone can see and harvest the same nodes (another player gathering from a mine won't deplete it for you, you can still gather there) there really is no reason to worry about finding the best path. What I usually recommend is to run around and gather all the Points of Interest/Vistas you haven't gotten yet while gathering. This will help you in the long run with 100% map completions. Two birds with one stone!
With out much more to be said about this location I will leave you with a list of the best sellers you will be finding here. Of course you'll find more items to sell than just the ones listed below, just be sure to check prices before selling them (a lot of the time the people buying items on the AH request to buy the item for less than vendor price).
Green Wood Log