Well, for one thing China has shut down their factories for the Year of The Dragon Holiday. Right now, companies that may be low on stock are selling a lot of after market parts -- this means a high amount of defective parts flooding the U.S. So, buying 5 cameras and only 1 working isn't a surprise.
On another note, check the flex cables connector and the receiving connector on the board and see if you spot any pins broke or bent.
Disconnecting and reconnecting these connectors will loosen and can easily lead to one breaking which can easily be overlooked.
I've also seen cases where connecting a new camera, while the iPod is powered up, can cause an electrical arc which will short/burn out the camera.