> there is no stock market that invests tens of millions of dollars in stopping people buying stocks There definitely are -- everything from margin limitation

> there is no stock market that invests tens of millions of dollars in stopping people buying stocks There definitely are -- everything from margin limitations, maintenance requirements, regulatory bodies, and counter party risk. > It doesn't improve efficiency in any way because the price you are taking is known. The price you are taking when you buy a stock is also known to all participants. Just like sports betting, the marketmaker (bookie) is profiting between the bid & ask spread.