Several decades ago federal laws were passed to keep people from manipulating markets. One way this was being done was to drive up costs by cornering the market, reducing the supply, and then slowly selling their supply at a far greater price than the original cost. The Bunker brothers did this with silver in the 1970s.
Another way to manipulate the market is by saying something which is untrue causing a stock or the market, as a whole, to move in the desired direction. For instance someone could short a drug stock and tell people a promising drug trial was a disaster causing the stock to fall. Doing the opposite would cause the stock to rise. If the manipulating information was a lie then it was illegal and the SEC could levy fines and restrictions. Federal and State Attorney Generals might file criminal charges too.
Market manipulation harms investors is when you and I are duped by the criminal actions. Market manipulation is being done on a massive scale today by our President. Yesterday, as in the past, Trump has made false statements to get the stock market to recover from previous gaffes or lies. He said the Chinese had contacted him, his staff at the G-7 meeting knew nothing about any call nor did the Chinese negotiators, and said they wanted to change their position. The Market rose.
When the President says things that lower the market, some people make money. When it goes back up with another statement more money is made. He often meets with hedge fund people who make billions in the market and heavily support him. Don’t you wonder if there is a quid pro quo connection?