This is not technically a scam and is totally game legal.
When TBC is released certain items will rise in price e.g. items needed to train jewelcrafting to 300, weps used by alliance shamen, horde pallys etc.
If someone sends you a cod item, you can leave that item unopened (unpaid) in your mailbox for several days before it is returned to the original sender (i think it is ten days)
Several days before TBC is released, use several alts to advertise constantly that they are buying say Star Rubys at a price that is 100%-200% of the current AH price.
Ask for the seller to send them to you cod.
Sit back and wait for TBC to be released.
If prices of the items rise, pay the cod price and relist them on the AH at the new TBC price.
congratuklations you just made a profit and the original cod seller just paid for the risk element since if prices dont rise, or fall when TBC is released, you simply return the cod items unpaid !
Trick here is to use a lot of alts and try and entice people that have stockpiled mats or epics for shamen etc to sell to you (cough) befiore TBC goes live - a certain percentage of stockpilers will be happy to simply make (they think) a profit on their items and hold gold when tbc is released, others will simply be happy to sell at + AH prices.
I am not sure what the limit of mails you can have waiting to be opened but it is certainly sufficient for you to potentially make a fortune on this "scam"