The problem with trying to profit from gathering a large quantity of them and manipulating the market is you can never be sure how much of your pile you will be able to get out of at the higher price.
It could end up like when the Hunt brothers tried to corner the silver market. http://en.wikipedia.org/wiki/Silver_Thursday By buying up most of the available supply they drove the prices higher, but when the time comes to unload your hoard at those higher prices the volumes are typically not there to support unloading the whole amount, and you end up driving the price back down trying to get out of your position.
The cartel effect works much better if you are a natural supplier of whatever you are trying to manipulate (like OPEC) so you can control the supply more effectively.