I just don't get why anybody who has successfully built one would ever sell one as they can make far more from keeping it than they ever would from selling it.
By selling to others they would be making their own machines less profitable, as soon as you have one you would soon be able to afford more.
problem being: if you keep them all to yourself you'd soon take over more than half of bitcoins hashing power, thus killing bitcoin and your own operation with it. if you keep some for yourself and sell of the rest you can make a profit both mining AND selling the hardware, thus getting ahold of more bitcoins without overtaking the network.