Bitcoins price is based on supply and demand on the market.More demand higher price more supply down price.Yes you can manipulate bitcoins price if you hold a huge supply or supply that may be enough to sell or buy in the current market status
Even if you hold a huge pile of bitcoins then also manipulating is not that simple because you create a sell order or 10 bitcoins at cheap price means that an equivalent buy order would be filled and hence not changing the market.
Yes, if you own very very huge amounts like on thousands of bitcoins then you can surely alter the market and even make it swing both ways.