They could purchase it for $500 per Bitcoin.
Then, due to the hightened demand and lack of any other way of getting bitcoin, they could be sold on for more than $1000. Making a $500/unit increase.
The price changes are much more dynamic.
The price of bitcoin would not remain at $500 once this big player starts buying - it will start to increase straight away.
Similary, the price will start to decrease once he stops buying and decrease even quicker, once he starts selling.
this mean that they would sell at loss, because someone else will ride their wave and dump before them, so this whole strategy to buy a lot of coins in a short amount of time is stupid
no group or single person will do it