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Why do people sell when demand pushes share price up?
If the price has gone up from what it was when the person bought, he may sell to collect his profit and spend the money. If someone intends to keep his money in the market, the trick is that you don't know when the price of a given stock will peak. If you could tell the future, sure, you'd buy when the stock was at its lowest point, just before it started up, and then sell at the highest point, just before it started down. But no one knows for sure what those points are. If a stockholder really KNOWS that demand is increasing and the price WILL go up, sure, it would be foolish to sell. But you can never KNOW that. (Or if you have some way that you do know that, please call me and share your knowledge.)