DETROIT — Whether it's a matter of ignorance or greed, people are still buying General Motors stock, even though the company and the government have warned that the shares will someday be worthless. Investors are picking up millions of shares every day, thinking they'll profit from what is really a hodgepodge of outdated factories and a pile of debt left behind when the new General Motors Co. exited bankruptcy court protection. Instead, they could end up losing money very quickly. The price of the shares, currently under $1, has ratcheted up or down as much as 50 cents in one day. On Wednesday, investors bought or sold 12.6 million shares for around 90 cents apiece.