This is for Wotlk+ expansion.
1)Buy from ah Frozen Orb (on my server the price is 20-30g)
2)Go to NPC
3)Exchange Frozen Orb on Eternal fire (on my server the price is 40-60 g) or other item from NPC which costs more on your ah.
4)Profit on one item you can earn 10-30g or more. If you buy more you could have better profit.
On 10 items you can earn 300g but demand on Wotlk eternals can be little.