The market maker first must acquire the shares (or short them) in order to sell in quantity.  This typically prevents such actions to a large degree.  The number of available shares is relatively small, so until they can scare investors out of their shares, it is difficult for maker makers to buy on the cheap.  One way MM's try to scare investors is by planting "Bashers" on the message boards where investors are likely to visit.  While this practice does cost money (wages), they often come out ahead if the right price targets are met.