This is not manipulation. If someone owns 18K in bitcoin and wants to sell, that is their choice and there is nothing anyone can do to stop them. If there was a way to stop a sale then I would be selling all my coins as well.
If you want to see a higher valuation of bitcoin then you are going to have to do something other than trading crypto for fiat, back and forth again and again. All that does is create wild swings in price and favor the person with a lot of coins. This is the economic model WE built and it is not unfair for someone to profit from that model.