Well remember there are more than 2 people investing. Millions of people/entities are investing/trading stocks. Liquidity comes in to play here. Also they don't have to be sold or bought. Someone could potentially own the whole float. The float is the amount of trade-able shares available. There are instances of stocks with extremely small floats being able to run or drop large percentages in very short amount of time if a large buyer or seller comes along.

The SEC frowns on manipulating penny stocks. One could own a large percent of the float or control shares that have not entered the float yet. (Example: converting a convertible debt note into shares) They could control the supply. This will get you in trouble these days if/when that penny stock is a shell with press releases touting false numbers and deals are being printed. LIFS was halted Tuesday and NVGC was halted Wednesday for essentially being pump and dumps; as examples.