If Bitcoin price pumps from $1300 to $3000 in few hours, nobody would make profit from it, because the exchanges wouldn't allow you to sell your Bitcoins and make the fast profit, it would bring a big loss for exchange sites.
Do you agree with this?
The exchanges do not buy your bitcoin with their own money but these exchanges are working as a middleman between the buyer and the seller and at that time when someone will sell their coins so someone will also buy bitcoin because they will neeed bitcoin for their business and so the sale will happen at that time.