There seems to be a lot of market manipulation going around by very rich people with thousands of gold, so I have the following suggeions.
1) Make things you bought on the trading post temporaly account bound (say 24h) and make it impossible to sell old stocks (not account bound) of that item for the same duration.
2) If you sold an item make a cooldown on it until you can buy more of the item.
3) Add an extra 10% listing fee + 5% sale tax for items you previously bought on the trading post (lets say for the last week).
