Off course everyone who has bitcoins can manipulate the bitcojn price but what you have to do is work together becuase you wnont change the price by your self you need
to sell all together and the price will rise for sure!
I think you said something wrong. selling bitcoin all together won't rise it's price.. but buying bitcoin will rise the price of bitcoin. if all people become together and sell all their bitcoin then bitcoin price will dump so badly.
The demand of bitcoin will make the price rise because some people may not be selling their coins which would increase the price. The law of supply will be applied here as the total bitcoins in the circulation are only limited.