anyone can sell a stock. they just have to deliver. even market makers who short a stock have to deliver. when they can't make delivery they are 'bought in' this means that DTC sends out notices to the various firms that they must go out to the street and buy the stock, at whatever the price may be, then whoever is short a stock is no longer short. someone else may be short, but then they have the risk of a buy in as well. now if someone corners a stock, (buys all of it), they have to file a 13D saying they own more than 5% of the stock. if the company buys all the stock back, they have to say that they are doing just that. If there is an offer, (ask) then someone is willing to sell it.