This is as much rumormongering as posting "leaked patch changes" is. Don't do it unless you have proof.
Market manipulation have in the past sometimes resulted in suspension and other Blizzard response.
The scale on which you need to subverse the game economy to render punishment is as far as I know not yet determined.
Neither do I know if Blizzard proactivly studies ingame economy or if they rely on player reporting before acting.
So the "buy a market niche and bleed it dry" has more disadvantages than the undercutters and supply flow control.
If anyone who have been subject to such sanctions would like to share here it could perhaps sevre as a good guideline on where the line is drawn and who draws it.