First off make sure you have a spare account and station it in Greenmont near the AH and Bank/Mail. Name it something like (yourname)Bank or something easy to remember.
Next off the most useful thing to focus on is a material as they sell quickly as many people want them for crafting but I wouldn't really focus on that unless you have a high crafting level.
I sell ores as they are quick to collect. And sell for quite abit.
Next send ore to Bank character and look at the prices on the AH always check the prices of them and look at the lowest reasonable price. if it is 2G then sell it for 1G 99S as this will make it more likely to sell.
Next sell the Items in stacks of there max value. always look on the AH for a price which is much lowerthan the others and buy it!You will be able to sell it for more than price so if the norm value is 4G and a stack is worth 2G buy it and sell it for 2G 99S as you will make a profit!
-Always sell in stacks for top amount of stacks as this will be more likely to sell.
-Check prices before making a price.
-work in 99S instead of 1G ( instead of 50S use 49S)