Enchanting Materials
skill that tailors will be able to sell to other players when Cataclysm rolls around, and it's the reason so many WoW players are leveling Tailoring right now. This means cloth is in demand, which should drive the price of cloth on the auction house up.
Another important way to farm for gold by instance leveling is through enchanting materials. Most of the time you will have an enchanter in your group, and you'll know if you do because the "disenchant" option on the loot bar will be available when everyone is rolling on gear drops. Always choose "disenchant" if you don't plan on using the gear. This will allow you to roll on the materials that come from disenchanting the item, allowing you to take advantage of the enchanter's skill to disenchant items. Enchanting materials will always sell for top dollar on the auction house, so make sure you save up stacks of them to sell as you're instance leveling.
Other Rare Materials
Instance leveling also gives you more opportunities to roll on other rare items that can be used for various professions. The trick with these items is being able to recognize which ones are worth gold on the auction house. In general, items that appear with a white name can be sold on the auction house because someone can use them for a profession. However, part of this is going to be trial and error. If these items repeatedly do not sell, then you're better off selling them to a vendor.