buying $10K worth of common stock won't bring down the system, nor create a monopoly. It's when you want to buy ALL the common stock (or at least 51%) of a company similar to the one you mentioned AND you own a rival chain is when the problems start. At that point, leveraging off your increasing assets base, you then go and buy ALL the hardware chains in the US and (work with me here) the world and all the (say) nail manufacturing plants. You can then charge what you want for a nail, if all nails are $10 each (say) in your hardware chain and you have no competitors: monopoly
You grow rich at the expense of the people. That's what Capitalism's ultimate goal is - monopoly. That's why Govt. regulates very strictly re super-mergers and takeovers.
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Bringing the "gift" of democracy to Iraq via invasion is akin to bringing the "gift" of Christianity via the Crusades - which worked really well, n'est pas?