So you're permitted to trade items "for free" but not if real-world money changes hands!?
Err, what business is it of Blizzards whether real world remuneration takes place in such trades? Either permit them or don't but whether money is involved is **** all business of theirs IMHO.
Because the items belong to Blizzard, not you. Thus, you cannot charge money for real life gain for an object that belongs to them.
They state this quite clearly in the T+Cs. All characters, items and other ingame paraphenalia are the property of Blizzard and may not be sold blah blah blah. I don't remember the exact words, but that's the jist of it.