Actually, what this whale (s) could do is put like 10k coins on stamp, and bring them into stamp at any time that s/he wishes, since BTC is so easy to move, and push the price down on stamp while accumulating coins on other exchanges and through other channels.  Therefore continuing to profit and continuing to be able to accumulate more coins than he is selling on stamp.  Something like that.

I don't really buy the theory of selling into your own buy wall on the same exchange to be profitable or even reasonable conduct.