Not sure if this is and exploit exactly but i just tought this was too good to be missed.
While the Harvest Festival lasts you can buy Green Fireworks from a vendor outside Ironforge from an NPC called Wagner Hammerstrike or outside Orgrimmar from an NPC called Javnir Nashak. What's the catch ? They cost only 20 copper and for every 36 of them you turn in at the darkmoon faire you get 8 tickets. That's only 7.20s for every 8 tickets, how cheaper can it get ? Get them fast while the Harvest Festival lasts