Define "dumps."  If you put in a market price order for a million dollars, the price goes through the roof and you get brutally raped by buying bitcoins at ever-increasing prices until the market runs out of sellers or you run out of money.  If you put in a bid for a million dollars at a specific price that's below the market price, you might influence the price towards that price, but you might not.  If you put in a bid for a million dollars at a specific price that's above the market price, the market price would immediately bump up to exactly your price, and you'd have bought however many bitcoins were outstanding with Asks below that price, plus have a giant outstanding order left over.