Sorry if this has been posted already... there are soooo many scams (most of them bannable) on this forum O_O
I haven't tried this yet but I have blue confirmation that this is officially NOT considered a scam.
Basically, what you do, is arrange to sell an item with someone else. In the case presented on the wow customer service forum, the victim had arranged to buy an essence of undeath for 1g. The scammer sent it via COD, but instead of setting it to 1g COD, he set it to 15g COD. The victim was careless and simply accepted.
edit:
Pavonum:
"Hello, Merkeen! I'm afraid that, unless the item is wrapped and it is unclear what you are accepting for the COD, what you have described is not a "scam." As it sounds as if you were able to see that the item in question was an Essence of Undeath, the player who sent you the COD has done nothing in violation of our policies."