You wont change price by buying and selling back. Because neither supply nor demand have changed. You can keep prices up by buying and hoarding over a long time

You wont change price by buying and selling back. Because neither supply nor demand have changed. You can keep prices up by buying and hoarding over a long time, but it means locking up capital in something that doesn't generate return... And wait, this has been tried in 2008 :) didn't end well especially for investors because no Ponzi scheme can run indefinitely. Eventually the stream of stupid money runs out because there are no bigger fools left, and to generate returns you need to sell, and if you sell, market falls.