If you're interested in a Tabard of Frost, a Fireworks Trinket or an Ogre Suit code from the WoW trading card game, you'll be looking at buying a lot of packs of WoW TCG cards for the point cards that come one to a pack.
Naturally, you save quite a bit of money just buying the point cards individually on eBay. That, I think, is common sense.
However, what a lot of people don't know is that the UDE point cards are included with other card games from Upper Deck and can be used to purchase in-game WoW items.
Frequently, I've seen Marvel Comics Vs. System UDE point cards and DC Comics Vs. System UDE Point Cards go for a lot cheaper than their WoW counterparts. You may even be able to score a box cheap as I don't think those games are anywhere nearly as popular as the WoW TCG.
Back when the Ogre Suit points cost $80-100, I got mine spending $30 on cards and probably could have gotten in cheaper had I waited and poached the best auctions. I think these go for around $60 now but it varies.
Once you have the code, you could sell it for real money. (Actually, Blizzard would RATHER you sell it for real money than in-game gold.)
So it's a cheaper way to an Ogre suit and possibly a way to make a decent profit legitimately eBaying codes.