OK Guys I see alot of people asking how to make Diamonds so i have a few things to say on how to help.
1: Storage. You need bag space and bank space to stockpile your goods. Sometimes making a new character can lead to more bag space and bank space. But more toons also increases the amount of Auctions you can post at a time so the more the better.
2: Finding a market. You have to pick what items you can buy and resell that will allow you to turn a profit.
3: Storing your AD on the Zen exchange, When Zen is low buy it and hold it and wait. this also allows you to move your AD from toon to toon when needed.
4: Buy low and sell high. Place Bids and pick up items a tad cheeper than buyout prices. Or buy at the buyout prices and push the price up on the item and then repost.
5: Farm your AD and buy your items for resell.
6: Get VIP. Preferably to the point where your Tradehouse fee's are removed. allowing you to keep your profits.
7: Use a little real money to get things started.
Some markets i would suggest, The Novelty market such as fireworks and the like. its over saturated now but if you have the storage you can make some money off these seasonal items later on down the road. Dyepacks are nice as well. Rank 5 enchants can be profitable as well.
Sometimes BUYOUT isnt the best way to sell some items. waiting 24 hrs 3days and 5 days for a sale will net you a little more money if it gets a buyer. selling in bulk helps this process,
I hope i have helped by stating the obvious. Good luck