Retail is a big deal these days. Used to be sub 10%, now it’s in the 30-40% of daily volume range IIUC.

You can easily look up the numbers you are asking for, the TLDR is that the volume in most stocks is high enough that you can’t manipulate it much. If it’s even 2x overpriced then there’s 100m on the table for whoever spots this and shorts, ie enough money that plenty of smart people will be spending effort on modeling and valuation studies.