Both operations are already illegal. The first one is market manipulation and punishable by law. It is interesting to note that this kind of manipulation occurs much more on the buy side ? called “long” ? (for instance, false takeover rumours), yet no one in his right mind thinks of forbidding the purchase of stocks. The second one is also prohibited, although many brokers allow it, since the penalty is very small and so the “crime” pays off. And yet this is a point that deserves supervision and the heavy hand of regulators.